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LIBRARY OF CONGRESS. 

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UNITED STATES OP AMERICA. 



2 1885 



LABORATORY EXERCISES 



PHYSICS. 



ALFRED P. GAGE. A.M.. 

Author of "Elements of Physics" and "Physical Technics.' 



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t >o i r 7 -* 



BOSTON: 
PUBLISHED BY THE AUTHOR. 

1884. 



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Entered according to Act of Congress, in the year 1884, by 

ALFRED P. GAGE, 
in the Office of the Librarian of Congress, at Washington. 



J. ^^^^^^^^ XGn STREET ' B ° STCN - 



PEE FACE. 



T~T has been our aim to collate in this volume something of 
value to every teacher of Physical Science. For valuable 
suggestions and aid in our efforts, we are deeply indebted to 
Prof. W. LeConte Stevens of Packer Institute, Brooklyn, N.Y. ; 
Prof. M. B. Crawford, Wesleyan University, Middletown, Conn., 
and Rev. J. G. Griffin of Ottawa College, Canada. Mr. Arthur 
W. Goodspeed of Harvard University prepared the key to the 
solution of problems. Messrs. J. S. Cushing & Co., Boston, 
are entitled to the credit for the excellence of the typography, 
and Messrs. Berwick & Smith, Boston, for the presswork. 

AUTHOR. 



CONTEXTS. 



PART I. 

PAGE. 

Laboratory Exercises 1-84 



PART II. 
Manual of Manipulations 85-129 

PART III. 
General Review of Physics 130-158 

PART IV. 
Test Questions 159-179 

PART V. 
Key to Solution of Problems 180-200 



Paet I. 

LABOEATOEY EXEECISES. 



PROPERTIES OF MATTER. -DYNAMICS. 

GRAPHICAL METHOD OF REPRESENTING 
VARIABLE QUANTITIES. 

Suppose that we have two quantities, x and y, so related to 
each other that any change in one alters the other ; for example, 
let x represent the interest, compound or simple, for a term of 
y } r ears. Take a piece of engineer's paper, Fig. 1, divided into 
squares by equidistant vertical 
and horizontal lines. Select one 
of each of these lines to start 
from. The vertical line is called 
the axis of Y, and the horizontal 
line the axis of X, and their 
intersection the origin. Take 
point a as the origin, and let 
the horizontal spaces to the right 
represent the number of years, 
and the vertical spaces multiples 
of the first year's interest ; then 
points a', a", a'", etc., represent 
the interest at the end of the 
first, second, third, etc., years. 
Connect these points by a straight line aA. Now, if we take 
any point, as c, in this line, and connect it with the axis Y by 
a horizontal line wc, and with the axis X by vertical line mc, 
the former will represent the time and the latter the interest 























B 








































































A 
































































/ 


/ / 
























(f 










" 












/ / 














'/ 




















I 


'// 




















I 


'/ 




















I 


/ 






















/ 






















/ 























Fig. 



2 LABORATORY EXERCISES. 

which has accrued at that time. The line nc is called the 
abscissa, and the line mc the ordinate of the point c. In a 
similar manner are found points 6, &', b", etc., representing the 
compound interest at the end of the second, third, and fourth 
years. Having connected these points by the curved line aB, 
the ordinate of any point in this line will represent the com- 
pound interest which has accumulated at the time represented 
b}' its abscissa. What does the point A represent? the point 
B? the line AB? 

Experiment 1. Construct a number of curves representing 
familiar phenomena, as the changes of temperature during the 
day or year, barometric changes, the velocity of a falling body, 
the declination of the magnetic needle at New York City (see 
p. 83) from 1680 to 1880, volumes of a given amount of air 
corresponding to different pressures, etc. 

CRYSTALLOGRAPHY. 

Exp. 2. Make saturated solutions of various substances, such 
as ammonium chloride, ammonium oxalate, potassium nitrate, 
cuprum nitrate, potassium bichromate, ferrum sulphate, barium 
chloride, urea dissolved in alcohol, etc., and flow slips of well- 
cleaned glass with each of these solutions. Allow them to 
"drain for a few seconds, and then, with a microscope or common 
magnifying glass, watch the growth of the crystals as they form 
upon the glass. 

Exp. 3. If the teacher possesses a porte-lumiere, or a stere- 
opticon, he should project the above on a screen by using the 
wet slips of glass as he would use the ordinary stereopticon 
slides. If the growth of crystals is slow, as is apt to be the 
case when the liquids are not fully saturated, it will be well to 
warm the slips of glass by waving them over a Bunsen or alcohol 
lamp flame, and then pour the solutions upon the warm glass. 

It would be well to encourage pupils to collect cabinets of 
cr3 r stals. The cn~stals should be preserved in small, well-stop- 



PROPERTIES OF MATTER. — DYNAMICS. 6 

pered bottles or test-tubes. Single crystals may be mounted 
on heads of pins, the points of the pins being thrust into cork 
so as to hang from the same inside the bottles. 

Exp. 4. Examine with a common magnifying glass a freshly 
broken surface of cast iron, and observe the crystalline appear- 
ance of the fracture. 

Exp. 5. Make a cold saturated solution of table salt, and 
allow it to stand several days. As the water evaporates, small 
crystals of salt will be formed. Make drawings of crystals of 
all the substances used. 

VISCOSITY. 

Exp. 6. (By Sir Wm. Thomson.) Take a cake of shoe- 
maker's wax, 18 inches in diameter and 3 inches thick, and 
place it in a shallow cylindrical glass vessel. Below the cake 
place a number of corks, and on top of the cake some lead 
bullets. Fill the glass vessel with water to prevent great vari- 
ations in temperature. In about a year's time the corks will 
float up through the wax to the top, while the bullets sink to 
the bottom, showing the viscous nature of the wax. 

Exp. 7. Take a strip of sheet lead about 40 cm long and 2 cm 
wide, and attach to one end by means of a clamp of some kind 
a weight of about 300 g . Support the whole in a vertical posi- 
tion b}' means of another clamp applied to the upper extremity, 
and note at regular intervals of time, by means of a fixed scale 
placed beside it, the elongation which has taken place. Draw 
curves of viscosity, the ordinates marking the elongation and 
the abscissas the units of time. 

Exp. 8. Determine the viscosit} T of glass. Get a glassblower 
to make a coil about 6 cm in diameter and 20 cm long from a piece 
of glass tubing about 1.5 m long and 6 mm in diameter. Suspend 
the coil in a vertical position, and attach to the lower extremity 
a weight of about 20 g . 

Exp. 9. Determine the viscosity of wires of different metals 
wound into coils, and draw curves of viscosity for each. 



4 LABORATORY EXERCISES. 

SOLUBILITY. 
Exp. 10. Compare the solubility in cold and in hot water of 
alum, saltpeter, common table salt, white vitriol (zinc sulphate), 
etc. In each case take 10 cc of cold water in a test-tube ; 
pulverize the solid to be tested, weigh out 50 g of it, place a 
small quantity in the test-tube, cover the mouth with a finger, 
shake well, and observe the rapidity with which it is dissolved. 
Continue to add small quantities at a time (smaller as it ap- 
proaches the saturated point) , as long as it is dissolved ; and, 
when saturated, weigh the remaining solid. Its weight = a 

Then — ^— = x, the solubilit}- of the given substance 



10 



grams 

in cold water. 

Exp. 11. Next suspend the test-tube in boiling water, and 
continue adding small quantities of the substance till saturated. 

50-6 



10 



Weigh the remaining solid ; its weight = b. Then 

the solubility of the given substance in water almost boiling. 
Prepare blanks, and record your results as follows : — 





Name of Substance. 


Solubility in Water. 


Cold. 


xVlmost Boiling. 


Alum 


0.15 


3.6 



The solubility of a substance is the weight in grams of the 
solid which 1 gram (l cc when water is used) of the solvent 
requires to form a saturated solution. In stating the solubility 
of a substance, ought the temperature of the solvent to be 
given ? What weight of alum will 40 cc of cold water dissolve ? 



PROPERTIES OF MATTER. — DYNAMICS. 5 

What weight will the same quantity of hot water dissolve ? How 
does the solubility of alum in hot and cold water compare? 

Exp. 12. Ascertain approximately the solubility of the sub- 
stances used in the last experiment in cold alcohol. 

Exp. 13. Put a granule of gum mastic in a test-tube con- 
taining water, and shake. Do the same with granules of iodine 
and analine. 

Exp. 14. Repeat the last experiment, using alcohol as a 
solvent. 

Exp. 15. Pour the solution of mastic obtained in the last 
experiment into a tumbler of water. 

Exp. 16. To 4 CC of a concentrated solution of sodium sul- 
phate add 2 CC of alcohol. 

In which are organic substances — i.e., substances of animal 
or vegetable origin — more commonly soluble, in water or in 
organic solvents such as alcohol ? Can one liquid diminish the 
solvent power of another? 

ABSORPTION AND DIFFUSION. 

Exp. 17. Take about l cc of ammonia water in a test-tube, and 
immerse the lower end of the tube in hot water ; in about a 
minute close the mouth of the tube with the thumb, invert it in 
a vessel of cold water, and remove the thumb. 

Exp. 18. Make a paste of plaster of Paris about l cm in depth. 
Take a glass tube 20 cm long and 2 cm in diameter, and thrust one 
end vertically into the paste, and hold it there until the paste 
hardens. Allow the tube to stand for a day or more to allow 
the excess of water in the plaster plug to evaporate. Hold the 
tube vertically, with the plugged end upward, and introduce a 
rubber tube connected with a gas jet, and fill with illuminating 
gas. Thrust the open end just beneath the surface of water, 
and hold it there for a few minutes. The water will gradually 
rise in the tube in consequence of the osmose of the gases. 

Exp. 19. Pour into a saucer about 5 CC of ether or bisulphide 
of carbon, and notice the rapidity with which its vapor diffuses 



h LABORATORY EXERCISES. 

through the air in a room, its presence being recognized by the 
sense of smell. The molecules issue from the bottle with great 
velocity ; and, if their progress were not interrupted by striking 
against the air particles, the room would be instantaneously 
permeated by the odor. 

DENSITY, ETC. 

Exp. 20. Prepare a concentrated solution of common salt, sul- 
phate of soda, or sulphate of zinc. Introduce first into pure 
water, then into the solution a wooden demonstration hydro- 
meter like that described in § 64, Physics, and, noting the 
depths to which it sinks in each liquid, calculate from the data 
obtained the den sit}' of the solution. In doing this, the pupil 
will intuitively grasp the philosophy of the hydrometer. 

Exp. 21 . Place an ordinary heavy-liquid hydrometer in water ; 
then add gradually (as fast as it will dissolve) powdered sul- 
phate of soda, stirring with a glass rod, and note from time to 
time the density of the liquid. Draw a curve of density repre- 
senting the density by abscissas, and the number of grams of 
solid dissolved by ordinates. 

Exp. 22. Place a light-liquid hydrometer in cold water, then 
in water at a temperature of about 80° C, and note the density 
of the two waters. Wiry must the standard of specific gravity 
be given at a definite temperature ? 

Exp. 23. Measure the capacity of some small cavity. First 
weigh the article containing the cavity, then weigh the same 
with the cavity filled with mercury, divide the difference between 
the two weights by the specific gravity of mercury, and the 
quotient will be the capacity in cubic centimeters. 

Exp. 24. According to the principle of parallel forces, the two 
arms of a balance beam ought to be precisely equal ; otherwise, 
unequal weights will be required to produce equilibrium. Test 
your laboratory balances by placing weights in the two pans 
until the beam becomes horizontal. Then interchange the con- 



PROPERTIES OF MATTER. 



- DYNAMICS. 



tents of the pans ; if the beam remains horizontal, the arms are 
equal, otherwise it will descend on the side of the longer arm. 

Exp. 25. Paradox. Take a strip of tin 50 cm in length and 6 cm 
in width, and bend it into the form of a circular hoop ; solder 
the two ends together. At some point in the interior of the 
hoop solder a lump of lead weighing about half a pound. This 
hoop may now be placed upon an inclined plane in such a posi- 
tion that it will apparently roll up hill. 

Exp. 26. Bore holes about 2 mra in diameter with the point of a 
pen-knife blade in the opposite ends of a hen's egg ; blow the 
contents out. Drop pulverized rosin through the hole in the 
large end so as to cover the interior surface of the small end ; 
then pour melted lead through the same hole, so as to " load " 
the small end. In whatever way you place the shell, it will 
stand on the small end. 

Exp. 27. Take a long glass tube (the longer the better), closed 
at one end with a tight-fitting cork, fill* it with water, and sus- 
pend it in a vertical position b}' a light spiral spring from the 
ceiling. Suspend at the top of the water column a number of 
bullets attached to the tube by a thread. With a flame, burn the 
thread ; during the descent of the bullets through the water, the 
spring contracts and the tube rises. 
Account for this phenomenon, and 
make a practical application of the 
principle involved. When any por- 
tion of our atmosphere ascends, in 
consequence of a denser portion de- 
scending, how will the pressure of 
the lower strata be affected ? 

Exp. 28. Prepare a Y-shaped bar 
like that shown in Fig. 2, the bar 
AC being about 3 feet long ; place it so that the end will 
overlap the table two or three inches, and hang a heavy weight 
or a pail of water on the hook B, and the whole will be sup- 
ported. Rock the weight back and forth by raising the end C 




Fig. 



8 LABORATORY EXERCISES. 

and allowing it to fall. What kind of equilibrium is this? 
Remove the weight, and the bar falls to the floor. Why? 

Exp. 29. In the small end of an egg make a hole about 2 mm 
in diameter, and place it, the small end downward, in a wine- 
glass, so that the end of the egg will be within l mm of the bottom 
of the glass. Place the whole under the receiver of an air- 
pump, and exhaust the air. The air which is contained in a 
small sack at the large end of the egg will expand and expel 
some of the contents of the egg. But, on re-admission of air to 
the receiver, the pressure of the air will drive the fluid back into 
the shell. 



EXPERIMENTS WITH EIGHT-IN-ONE APPARATUS. 

Exp. 30. Insert the stopper a (Fig. 3) in the base, and 
remove the caps 6, c, d, and e, and fill the cup /with water so 
that the liquid surface will be above the bend f/, and the lateral 
jyressure of liquids will be shown by the issue of liquids from the 
side orifices. (It may be necessary to remove temporarily the 
cap from h to allow the air in the tube to escape.) 

Exp. 31. That pressure increases with the depth is shown by 
the increase of velocitj' of the streams as the depth increases. 

Exp. 32. Remove the stopper a, and the liquid ceases to flow 
from the side orifices, showing that during the free fall of liquids 
there is no lateral pressure. «» 

Exp. 33. Replace the stopper a, and the caps on 6, c, d, and 
e, and remove the cap from /*, and connect with this tube, by 
means of a rubber connector, a glass tube i ; and, elevating the 
latter at various angles, the exact paths of projectiles at these 
angles are shown. 

Exp. 34. Remove the stopper a and the cap from j 9 and 
close all the other orifices ; connect with the tube j a glass tube 
fe, the lower end of which dips into a vessel of liquid m, and 
this liquid will be drawn up the tube, illustrating the action of 
the Sprengel pump. 



PROPERTIES OF MATTER. 



- DYNAMICS. 



Exp. 35. It will be seen that the receiver /is a Tantalus cup, 
and that here it is turned to a practical use, inasmuch as it will 
not suffer the liquid to flow until the vessel is full, and all is 
ready for the experiment. 

Exp. 36. It is evident that, when the cap 
is removed from h, and the tube i is suitably 
elevated, the instrument becomes a siphon- 
fountain. 

Exp. 37. The fountain ma} 7 easily be 
made to represent an intermittent spring or 
fountain by placing above the receiver a 
large vessel of water n, from which liquid is 
siphoned into the receiver /. The siphon 
delivery being smaller than that of the Tan- 
talus tube, it is evident that the fountain 
will operate intermittently and at regular 
periods, inasmuch as the liquid will not flow 
from the Tantalus cup until it is filled to the 
level of the bend, and will then flow until 
the cup is empty. 

Exp. 38. Place the apparatus from 8 to 
12 ft. above the ground, remove the caps 
5, c, d, and e, fill the cup /with water, and 
note the maximum horizontal distance, 
measured on the ground, which each stream 
attains. Draw a curve of pressure, the ordi- 
nates representing the distance of each ori- 
fice below the bend g, and the abscissas the 
horizontal distances attained respectively by 
the streams. Draw a curve of velocity on 
the principle that the velocity varies as the 
square root of the pressure or head of water, 
the ordinates representing the velocity and 
the abscissas the pressure at the several 
orifices. 




l;\ 




10 LABORATORY EXERCISES. 

Exp. 39. Stop up all the orifices but one, and note the num- 
ber of seconds it takes to empty the cup through that orifice ; 
do the same with each orifice, and draw a time curve represent- 
ing the time by ordinates and the distance below the bend g by 



Exp. 40. Provide glass tubes 20 cm , 40 cm , 60 cm , and 80 cm long, 
and of uniform bore ; connect them successively with the tube 7<, 
and note the time consumed in emptying the cup through each. 
Draw a curve of hydraulic friction, the abscissas representing 
the increment in length of tube, and the ordinates the increment 
in time consumed, or friction. 

Exp. 41. Keep the cup constantly full by pouring or siphon- 
ing water into it. Close all the orifices save one, the stream 
from which you wish to represent graphically. Determine the 
horizontal projection or random of suitable points of the 
stream by measuring perpendicularly from a line let fall from 
the orifice ; also determine the vertical distance of each point 
below the orifice ; then, by means of corresponding ordinates 
and abscissas, and on a scale of equal parts, construct a 
graphical representation of the stream. In similar method 
represent the paths of the streams from the several orifices. 

Exp. 42. Connect the glass tube i with h, elevate it to any 
desired angle, fix it firmly in this position, and make measure- 
ments similar to those in the last experiment, except that the 
vertical distances are to be measured upward from the orifice 
h ; construct from the data obtained a graphical representation 
of the path of a projectile directed at this angle. It will be 
found convenient to make all the horizontal measurements 
from the vertical tube ag, deducting therefrom the length of 
the horizontal tube. 

Exp. 43. Elevate the tube i so that it will be nearly vertical, 
and observe how much the stream falls short of reaching the 
orifice of the tube in the cup when the bend g is covered. 
If the stream encountered no resistances from the air, or 
from friction against the sides of the tube, how high ought 



PROPERTIES OF MATTER. — DYNAMICS. 



11 



it to rise ? If to the velocity of efflux from the orifice h there 
should be added the velocity which is lost in consequence of 
resistances, how would the sum compare with the velocity 
which a stone would acquire in falling a distance equal to that 
from the surface of water in the cup to the tube h ? 



EXPERIMENTS WITH THE "SEVEN-IN-ONE APPARATUS." 

Exp. 44. Suspend the instrument, Fig. 4, from some con- 
venient support, and rarefy the ah' within it by suction with 
the mouth ; a weight of 20 lbs. ought to be easily raised. 
This weight, added to a probable friction of about 15 lbs., 
gives 35 lbs. as the unbalanced pressure exerted by the outside 
air on the piston. 




Exp. 45. Push the piston quite into the cylinder, and close 
the stop-cock, and let a person grasp each of the handles 
and attempt to pull the piston out. It is apparent that in 
his attempt he creates a vacuum, and few will be found 
strong enough to draw the piston out. This constitutes a most 
interestiug modification of the classical Magdeburg Hemispheres, 
but with this important advantage, that it produces a self -cre- 
ated vacuum (the harder the pull the higher the vacuum) , and 
requires no air-pump. 



12 



LABORATORY EXERCISES. 



Exp. 46. Let a person alternately blow and suck air through 
the rubber tube (Fig. 5), and he will find it difficult to resist 
the forces (what forces ?) tending to move the piston. 

Exp. 47. Does the weight which is raised correctly represent 
the lung-power exerted, or is it a case similar to that in which 

a force is applied to the long 
arm of a lever ? 

Take a glass tube about G dm 
long and 5 mm bore, bend it 
into a U-shape ; pour into it 
mercuiy so that it will staud 
at a depth of about 15 cm in 
both arms ; blow into one arm. 
and the mercury will rise in 
the other arm. Measure the 
bight of the upper surface 
above the lower ; the weight 
of a column of mercury of an 
equal depth and the same di- 
ameter as the bore of the tube 
represents the lung power 
exerted. With a straight tube 
of the same bore, measure out 
such a volume of mercury and 
weigh it. 

Exp. 48. Remove the handle 
from the piston, invert the in- 
strument, place on the piston 
a block of wood, as in Fig. 6, 
and on the block a weight, 
and support the whole on a 
box. Blow into the instrument, and a heavy weiglit may be 
raised. The instrument thus becomes a pneumatic belloivs. 

Exp. 49. Connect with the rubber tube by means of the stop- 
cock another rubber tube, so that the whole Lmgth shall be 




PROPERTIES OF MATTER. — DYNAMICS. 13 

12 feet (Fig. 7). Elevate the tube, insert in the upper ex- 
tremity the funnel-shaped brass mouth-piece, and pour water 
down the tube, and a heavy weight may be raised by hydrostatic 
pressure, and thus the instrument becomes a hydrostatic bellows. 

TRIBOMETRIC MEASUREMENTS. 

Provide an oak plank 2 m long, 25 cm wide, 5 cm thick, planed 
smoothly on oue side ; also several sleighs with smooth sur- 
faces, as follows: A, an oak plank 14 cra x 14 cm x 2.5 cm ; B, 
ditto, 28 cm x 14 cm x 2.5 cm ; O, a pine plank of same dimensions 
as A. Attach a pulley to the middle of one end of the plank in 
such a manner that a string fastened to one end of a sleigh and 
passing over the groove of the pulley will be parallel with the 
surface of the plank. Provide also two pails and a half bushel 
of sand. 

Exp. 50. Place the plank in a horizontal position, about 2" 1 
above the floor, and on it sleigh A, near the end opposite the 
pulley. Place one of the pails on the sleigh containing sufficient 
sand to cause, together with the weight of the plank and pail, a 
pressure between the sleigh and plank of (say) 10 lbs. Carry 
the string attached to the sleigh over the pulley, and from the 
free end suspend the other pail. Place sand in this pail, reg- 
ulating the amount, so that, on starting the sleigh with a light 
blow, it will move along the plank with approximately uniform 
velocity. The weight of this pail, together with the sand, is the 
measure of the friction F caused by the given pressure P. 

The ratio of the friction to the pressure is called the coeffi- 

cient of friction ; i.e., — = O, the coefficient of friction. 

Exp. 51. Make P= 20 lbs., and ascertain F by experiment. 
Find the coefficient of friction, and compare it with that obtained 
in the preceding experiment. 

Exp. 52. Repeat the last experiment, using sleigh _B, making 
P the same as before. How does the amount of surface in con- 
tact affect the amount of friction ? 



14 LABORATORY EXERCISES. 

Exp. 53. P remaining the same, find the coefficients of fric- 
tion when the fibres of the sleigh are parallel to the motion, and 
when they are perpendicular to the motion. 

Exp. 54. With P — 20 lbs., and using sleigh C. ascertain the 
coefficient of friction between oak and pine. 

Exp. 55. Ascertain the friction of motion and of 
repose, i.e., the value of F necessary to keep up uni- 
form motion, and the value of F just sufficient to over- 
come the state of rest.. 

Exp. 56. Make the suspended weight great enough 
so as to produce a slightly accelerated motion, and de- 
termine whether the acceleration is uniform. If it is 
uniform, we must conclude that the amount of friction 
is independent of velocity. 

SIPHON BAROMETER. 

Exp. 57. With the blow-pipe flame close one end of 
a glass tube whose internal diameter is 6 mm or 7 mm , and 
length about 1.20 m . Apply the flame so as to make a 
slight constriction about 20 cm from the open end ; then 
about 6 cm or 7 cm farther, so as to bend the tube into 
U-form, the open arm being parallel to the closed arm. 
and distant from it 2 cm . If this be filled with mercury, 
with care lest any air-bubbles remain entangled, the 
difference of level ef between the two columns will 
be about 76 cm , being greater or less in proportion to 
the atmospheric pressure. 

Upon a strip of wood whose length be is l m , and 
^SSHL breadth dc is 5 cm , fasten a yard-stick eg or a metre- 
stick ; the latter is preferable, and should be placed 
' ' midway between the two edges of the strip. Any 
smooth strip of wood will do, if cut squarely off at the bottom 
e, and a mark / be drawn across at a distance of 7G cm from e. 
On each side of this mark the edge may be graduated in milli- 
meters for a distance of 5 cm or 6 cm . 



PROPERTIES OF MATTER. — DYNAMICS. 15 

Place the glass tube against the broad strip so that its lower 
curved portion may enclose the graduated strip. Secure it 
loosely with little loops of tiuned sheet-iron, 7*, i,j, k, so that 
the tube may slide easily past the fixed graduated strip. At the 
lower end fasten a little block of hf.rd wood bb', projecting 
enough to form a ledge on which the bend of the glass may rest. 
Through a hole in this ledge pass a tight screw, with milled 
head and smoothly -rounded end. The block serves as a nut, 
and the bend of the glass rests on the rounded end of the 
screw. The glass may be therefore lifted or depressed by 
turning the screw. 

Adjust the quantity of mercury so that the level in the open 
arm shall be tangent to the line forming the bottom of the 
graduated strip at e. This line may be extended across eZ, on 
paper, the lower half em being made black with ink, the upper 
half ek white, so that the hight of the curved surface of 
mercury at e can be accurately adjusted against a sharply-defined 
background. 

Finally, a screw-eye at g may be inserted, so that the instru- 
ment may hang vertically. 

If the atmospheric pressure should decrease, the column at 
/ falls and at e rises. By a few leftward turns of the screw, 
the tube and its contents sink down until e is brought opposite 
the fixed zero-point. If the pressure increases, a few right- 
ward turns bring e up to the zero-point. After adjusting at 
zero-point, the hight of the barometric column is read in the 
neighborhood of /. 



16 LABORATORY EXERCISES. 



HEAT. 

Exp 58. Conduction. Lay a thin gelatine card on the palm of 
the hand. The card being a poor conductor of heat, the lower 
surface will become more heated than the upper ; consequently it 
will expand more, and bend so that the ends will meet. If it is 
placed upon a cold surface it will bend in the opposite direc- 
tion. Glass (especially a variety called lime-glass) ware, when 
suddenly subjected to great heat, tends to bend in the same 
manner, but, being only slightly flexible, it is liable to crack, 
especially if it is thick, 

Exp 59 Wrap the bulb of the thermometer in muslin, and 
dip it into ether; allow it to remain until the mercury has 
acquired the temperature of the liquid, then take it out and 
note the fall of temperature. 

Exp 60. Take a deep beaker of about 800 cc capacity three- 
fourths full of cold water. Pour gently on its surface linseed 
oil at about 110° C, to the depth of about one inch, and suspend 
several thermometer bulbs at different depths in the liquid, and 
notice the slowness with which heat is conducted through the 

Exp. 61. Cut off the nose of a glass funnel of about 800 cc 
capacity. Pass the stem of an air thermometer (having a small 
bulb) down through the funnel, leaving the top of the bulb 
about five-sixteenths of an inch below the surface of a card laid 
temporarily across the edge of the funnel. Fill the space be- 
tween the neck of the funnel and the stem of the thermometer 
with sealing-wax, so as to become water-tight, Support the 
whole in a ring of a ring stand. Partially fill the stem of the 
thermometer with water colored with ink, and pour cold water 
into the funnel until its surface is one-fourth of an inch above 
the bulb. Then carefully pour ether upon the surface of the 
water until the funnel is full, being careful that none runs down 



HEAT. 17 

the side of the funnel. Ignite the ether with a match, and notice 
the slowness with which the heat from a very hot fire penetrates 
the quarter of an inch depth of water, so as to affect the sensi- 
tive thermometer. 

Exp. 62. Convection. Fill a thin glass flask of about 
400 cc capacit}' with hot water deeply colored with ink. Stop- 
per the flask with a cork pierced with a glass tube having a 
bore of about 5 mm diameter. Close the exposed end of the 
tube with a finger, and thrust the flask to the bottom of a tub 
or pail (preferably a deep glass jar) filled with cold water, and 
withdraw the finger. A stream of colored liquid will ascend 
for a long time from the flask to the surface of the water in the 
larger vessel. 

Exp. 63. Fill a glass flask as before, invert, thrust the 
neck into water, and withdraw the cork. No convection takes 
place downward. 

Exp. 64. With a porte-lumiere and lens, project a large 
circle of light on a distant screen, and suspend in the path of 
the beam, beyond the focus of the lens, a red-hot metallic ball 
or a candle flame. 

Exp. 65. Lay a block of ice across the back of two chairs, 
and over it pass a piece of fine iron wire, the ends of which 
have been twisted together. From the wire suspend as great a 
weight as the wire will support, say 25 to 50 lbs. It is evi- 
dent that since the extent of ice surface on which a fine wire 
will press is small, the pressure per square inch on the ice must 
be veiy great. The consequence is that just beneath the wire 
the ice is melted, and the wire drops down a little. As soon as 
the wire falls, however, the water about it is relieved from 
pressure, and immediately freezes. In a short time, therefore, 
the wire passes completely through the ice. 

Exp. 66. Take 200 g of fine dry ice chips or snow at 0° C, 
and an equal weight of water at 80° C. ; pom' the latter upon 
the former, and with a glass or wooden rod stir and melt the 
solid as quickly as possible ; and as soon as all is melted, take 



18 LABORATORY EXERCISES. 

the temperature. The experiment should be conducted in a 
room whose temperature is as nearly 0° C. as possible. 

Exp. 67. Take about a tablespoonful of water in a test- 
tube, twist around the tube near the bottom a wire for a 
handle, surround the tube with a freezing mixture, and freeze 
the water ; or a lump of ice ma}' be dropped into the tube, and a 
pebble-stone or coil of wire placed on top of it to keep it down. 
Then pour cold water into the tube, nearly filling it, and hold 
the tube in a flame (as in Exp. 5, p. 143, Physics), and boil 
the water at the top without melting the ice. 

Exp. 68. Place a beaker of water at about 40° C. under 
the receiver of an air-pump, and exhaust the air, and cause the 
water to boil. 

Exp. 69. Repeat the last experiment, causing the water to 
boil for about ten minutes ; at the same time expose to the tem- 
perature of the same room in another beaker an equal quantity 
of water at the same temperature. At the end of the operation 
take the temperature of both waters. Account for the differ- 
ence in temperature. 

Exp. 70. Do up a piece of writing-paper into a cone shape, 
gumming the edges smoothly down. Fill it with water, and 
place it in a loop of wire, and hold it in a Bunsen or spirit 
flame, and boil the water. The paper will not be charred, for 
a temperature of 100° C. is not sufficient. 

Exp. 71. Paste a strip of paper smoothly around a cylinder 
of brass or copper, and another strip around a wooden cylinder. 
Hold them in a Bunsen or spirit flame. The paper on the 
wooden cylinder will soon become charred, but the paper on 
the metallic cylinder will not char for several minutes because 
the metal conducts the heat away so rapidly from the paper 
that it is not readily raised to the ignition point. 

Exp. 72. Take two air thermometers, the bulb of one of 
them blackened with soot from a candle flame. Let the liquids 
in the two stems stand at the same hight. Expose each bulb 
for the same length of time to the sun's rays. 



HEAT. 19 

Exp. 73. Repeat the last experiment, except that a convex 
lens be interposed, and the bulbs placed in the focus of the 
rays. 

Exp. 74. Take two thermometers like those used in the 
last two experiments, aud fill the bulbs and a portion of the 
stems of each with hot water, and set them, stems upward, in a 
cool place, and observe by the fall of liquid in the stems which 
cools more rapidly. 

Exp. 75. Partially fill a bladder or the spherical rubber 
ball furnished for pneumatic experiments with cold air, and 
place it near a hot stove for a time. 

Exp. 76. Fill a test-tube of about 2 cm diameter with water. 
Insert, water-tight, a stopper pierced with a small glass tube, 
crowd the stopper into the test-tube so that the water will rise 
in the tube four or five inches. Surround the test-tube with a 
freezing mixture, and watch for the maximum density of the 
water, which is reached when the water in the tube reaches its 
lowest point. Subsequently the water will rise in the tube. 
This phenomenon is best observed through a telescope a short 
distance away. Or it may be projected by a porte-lumiere on a 
screen. 



20 LABORATORY EXERCISES. 



ELECTRICITY. 

INTRODUCTORY EXPERIMENTS. 

Experiment 77. Place a (tangent) 1 galvanometer so that the 
needle at both extremities points to zero on the graduated circle ; 
in other words, so that the coil will lie in the magnetic meridian. 
Connect the wires leading from a (Daniell or Bunsen) voltaic 
cell with the screw cups of the galvanometer. This is called, 
technically, " introducing a galvanometer into the circuit," in- 
asmuch as the galvanometer now forms a part of the circuit, 
and the current is obliged to pass through it. When the 
needle comes to rest, note the angle of deflection as indicated by 
the number of degrees to which either extremity of the needle 
(if there is a difference in the readings, take a mean of the two) 
points. Observe at which screw cup the current enters the 
galvanometer, and at which it leaves it, and the direction of 
the deflection. 

Exp. 78 » Remove the wires from the screw cups, and, by 
crossing them (if the wires are not insulated, they should not 
touch each other), insert each wire in the screw cup opposite 
the one to which it was before applied. The deflection of the 
needle is reversed. This shows that the direction of the current 
through the galvanometer has been reversed. But, by inspec- 
tion of the circuit, it will be seen that the current is reversed in 
only this portion of the circuit. Can you invent some arrange- 
ment by which the current can be reversed in a portion of the 
circuit more conveniently than by shifting the wires from cup 
to cup? Such a device would be called a pole changer or 
commutator. 

1 Two needles are furnished with each galvanometer : one is astatic, and when this 
is used the instrument will be called an astatic galvanometer ; when the other needle is 
used it will be called a tangent galvanometer. 



ELECTRICITY. 21 

Exp. 79. Take a voltaic cell, constructed in the manner de- 
scribed in § 151 of the Physics, first using an unamalgamated 
zinc. Connect each of the wires attached to the strips of copper 
and zinc with the galvanometer. Thrust them iuto the dilute sul- 
phuric acid, holding them quietly in it and at a constant distance 
apart. Watch for a few minutes the deflection of the needle. 

Exp. 80. Repeat the last experiment, using an amalgamated 
zinc. The deflection diminishes in time, but not so rapidly as 
in the last experiment. 

In the preceding experiment the local currents, established 
by the impurities in the zinc, did not pass through the main 
circuit ; consequently the main current was very weak. In this 
experiment, the entire current developed in the battery passes 
through the main circuit. The falling-off of the current which 
followed the first introduction of the strips into the liquid was 
due to the polarization of the strips. 



EFFECTS OF CONDUCTORS OF DIFFERENT LENGTHS, 
SIZES, Etc. 

Exp. 81. Introduce into the circuit with the galvanometer 
Spool 1 (see Catalogue of Apparatus), containing 32 yards 
of No. 23 copper wire, in such a manner that the current 
from the battery will pass through both the galvanometer and 
the spool ; it matters not which it passes through first. To 
make certain that the connections are all properly made, it is 
best for a young experimenter mentally to trace the current 
from the carbon successively through every connection and 
instrument, back to the zinc of the battery, and see that a suit- 
able path is opened for the current. The screw cups on each 
side of a given spool are to be used to send a current through 
that spool. 

A deflection smaller than before ensues. The circuit is now 
32 yards longer than before, and the result is a weakened cur- 
rent, resulting from increased resistance. Compare the currents 



22 LABORATORY EXERCISES. 

in this experiment with the current in Exp. 77, by compar- 
ing the tangents of the degrees of deflection as obtained from 
Table of Tangents, p. 403, Physics. Thus, suppose that tb? 
deflection in the first experiment was 84° and in the latter 80° : 
the tangent of 84°= 9.51; of 80°= 5.67. 9. 51-- 5.67 =1.6 +, 
i.e., the former current is about 1.6 times the latter. 

Exp. 82. Substitute Spool 3, containing 16 yards of No. 23 
copper wire, for Spool 1. A larger deflection is obtained than 
in the former experiment, and you learn that, other things 
remaining the same, the current varies inversely with the length 
of the circuit. It is proper to observe here, that if the entire 
circuit were made half as long, in other words, if the entire re- 
sistance in the circuit were reduced one-half, the current would 
be exactly doubled. In this case, only a portion of the circuit 
is reduced one-half ; consequently, the current is not quite 
doubled. 

Exp. 83. Substitute for Spool 3, Spool 2, containing 32 yards 
of No. 30 copper wire. A smaller deflection is obtained than 
with Spool 1, which contains the same length and kind of wire, 
but of greater diameter. Conclusion ? 

Exp. 84. Obtain the deflection with Spool 4, containing 16 
yards of No. 30 copper wire, alone in the circuit, and compare 
the current with that obtained in Exp. 83. 

Exp. 85. Obtain the deflection with Spool 5, containing 16 
yards No. 30 German silver wire, alone in the circuit. Compare 
the current with that obtained in Exp. 84. Conclusion? 

INTERNAL RESISTANCE. 

Exp. 86. Take a strip of copper and a strip of amalgamated 
zinc such as used in Exp. 79. Connect the free extremities of 
the wires with the screw cups of the galvanometer, and introduce 
the two strips into the liquid, keeping them about half an inch 
apart. Note the deflection of the needle. Raise the stripe half 
way, and three-fourths way, out of the liquid, noticing the cor- 



ELECTRICITY. 



28 



responding deflections. As the strips are raised out of the 
liquid, the size of the liquid conductor between them, in other 
words, the transverse section of the portion of the liquid between 
them, is diminished, and the result is an increase of resistance 
corresponding to the increase of resistance which attends the 
reduction of size of a solid conductor, as seen above. 

Exp. 87. Once more place the strips as at the beginning of 
the previous experiment, then gradually separate the strips as 
widely apart as the tumbler will admit, and note the correspond- 
ing changes in deflection of the needle. The farther the strips 
are separated the less the deflection, showing that the effect of 
increasing the length of a liquid conductor is the same as 
increasing the length of a solid conductor. 



MEASURING RESISTANCES. 

Exp. 88. Measure the resistance of the wire on each one of 
the spools as follows : First, introduce a spool into the circuit 
with a galvanometer, and get the deflection. Then remove the 
spool, and introduce in its place the rheostat or set of resist- 
ance coils. Place the 
three switches A, B, 
and C (Fig. 9) each 
on the zero butt of its 
corresponding gradu- 
ated arc. The circuit 
will then be closed 
through this instru- 
ment, as may be seen 
by the deflection of the 
needle of the galvano- 
meter. If one of the switches does not at any time rest on a 
butt, the circuit will be broken. By comparison with the de- 
flection when the rheostat is not in circuit, it will be seen 
that when all the switches are on the zero butts there is no 
appreciable resistance introduced into the circuit through the 




24 LABORATORY EXERCISES. 

rheostat. Now you are to aim to obtain the same deflection 
of the needle of the galvanometer that you obtained when 
the given spool was in circuit. This is done by introducing 
through the rheostat a resistance exactly equal to the resistance 
of the wire of the spool. You find on the upper surface of the 
box which encloses the coils (and thus protects them from in- 
jury) three graduated arcs, one extending from 0.1 ohm to 0.9, 
the next from 1 ohm to 9 ohms, and the last from 10 ohms to 
100 ohms. As in weighing with a balance beam and a set of 
weights of three denominations, you put into one of the pans 
weights until you succeed in balancing the article to lie weighed, 
and then add together the weights of the three denominations 
to get the total weight, so you introduce resistances into the 
circuit by moving one and another switch up their respective 
scales until the required deflection is obtained. Then add the 
resistances of the three denominations (corresponding to the 
tenths, units, and tens of the decimal system of notation) . and 
the sum is the resistance of the wire and the spool. This 
method of measuring resistance is called the method by substitu- 
tion. It is the most expeditious method, and for many practi- 
cal purposes gives sufficiently accurate results. It is of course 
based upon the assumption that the E.M.F. of the battery 
remains constant, — a consummation, as will appear farther on. 
rarely fully realized. 

Exp. 89. Introduce into a voltaic circuit through a rheostat a 
known resistance r, and obtain a deflection a. Then introduce 
in place of the rheostat a wire whose resistance r' is to be meas- 
ured, and get deflection a'. Since the tangents of angles of 
deflection are proportional to the currents, and currents are 
inversely proportional to the resistances, we have 
tana' : tana : : r : r' ; 

, , r tan a 

whence r = -• 

tan a 

From this formula compute the resistance of the wire. Verify 

the result by the " method by substitution." 



ELECTRICITY. 25 

Exp. 90. Measure the resistance of Spool 4, containing 16 
yards of No. 30 copper wire ; measure also the resistance of 
Spool 2, which contains the same kind of wire ; calculate the 
length of the wire on the latter spool as follows : 

r:r' ::l:V. whence l'= — , 
r 

in which I is the length of the wire in Spool 4, and r its resist- 
ance, r' the resistance of Spool 2, and V the length of wire to be 
found. 

Exp. 91. Connect in series from five to ten Bunsen cells, and 
introduce into the circuit with the batten 7 through a rheostat a 
resistance of (say) 50 ohms. Note the deflection. Remove 
the rheostat, insert the two poles in metallic handles such as 
are used in giving shocks. Let a person moisten his hands 
with a solution of salt in water, and grasp tightly the handles. 
Note the deflection, and by means of the formula given in Exp. 
89, calculate the resistance offered by the person's body. 

Exp. 92. Measure the resistance of a galvanometer as follows. 
Introduce another galvanometer into the circuit with the galva- 
nometer whose resistance is to be measured, and note the deflec- 
tion of the needle of the former galvanometer. Substitute for 
the latter galvanometer the rheostat, and measure the resistance 
in the same manner as you would measure any other resistance. 

Exp. 93. Measure the resistances of a voltameter, a piece of 
platinum wire, electro-magnets, either separate from other 
apparatus or such as constitute parts of apparatus, such as the 
electro-magnet of a telegraph sounder or a relay. 

Exp. 94. Take about l m of No. 30 iron wire, wind it into 
a coil about l cm in diameter, and as close as possible with- 
out allowing the turns to touch one another. Introduce the coil 
into a circuit with a galvanometer, and note the deflection. 
Then heat the coil very hot by applying Bunsen flames along 
its length, and note the diminution of current owing to increased 
resistance. 



26 LABORATORY EXERCISES. 

MEASUREMENT OF SPECIFIC RESISTANCES. 

Exp. 95. Take two wires of the same size and different 
material; e.g., copper and platinum, or copper andiron; get 
the deflection with one of them (e.g., the iron) in circuit, then 
replace it by the other, and carry one electrode of the battery 
along its length until the same deflection is obtained. The 
ratio between their lengths will express the ratio between their 
specific resistances. 

Exp. 96. Take two wires of different substances and differ- 
ent sizes ; determine the lengths which have equal resistances. 
Find, by a wire gauge, or micrometer screw gauge, the diame- 
ters of the wires, and then ascertain the ratio of their specific 
resistances by the formula 

q g'' 

in which s and s' represent their respective specific resistances, 
I and V their lengths, and q and q' the areas of their cross 
sections or squares of diameters. 

EFFECT OF CORRODED CONNECTIONS. 

Exp. 97. Find (they are usually not difficult to find) some 
piece of electrical apparatus which has a rusty or corroded con- 
nection, such as rusty screws of screw-cups, either connected 
with the battery or other apparatus. If these cannot be found, 
select a wire which has become corroded at one or both ends 
by exposure to acid fumes. Introduce this corroded connection 
into the circuit with the galvanometer, and note the deflection. 
Now clean all the rusty connections with a fine file, fine sand- 
or emery-paper, or by scraping them with a knife, and once 
more send the current through the same apparatus. The de- 
flection is now greater than before. Introduce through a 
rheostat a resistance sufficient to give the same deflection as at 
first, and thus measure the resistance caused by the corroded 
surfaces of contact. Beware of corroded connections. 



ELECTRICITY. 27 

JOINING CELLS IN OPPOSITION. 

Exp. 98. Take two cells as nearly alike as possible, connect 
the zincs of each cell with each other by a short copper wire, 
and the two carbons with the galvanometer. There is either 
no deflection, or, at most, a very small deflection. This is 
called joining cells in opposition. Here the tendency of one 
cell to produce a current in a certain direction is counterbal- 
anced by the tendency of the other cell to produce a current 
in the opposite direction, and no current flows, provided the 
E.M.F. of both are equal. If the E.M.F. of both are not 
equal, then there will be a current proportionate to their 
difference. 

ELECTRO-MOTIVE EORCE. 

Exp. 99. Connect, in opposition, two cells of the same kind 
but of different sizes ; e.g., a quart cell with a gallon cell ; or, 
which amounts to the same thing, lift up the zinc or the carbon, 
or both, of one of the cells, thereby diminishing the immersed 
part of one of the cells ; no deflection ensues. It thus appears 
that a large cell has no greater power to produce a current than 
a small cell; i.e., the E.M.F. of battery cells does not depend 
upon their size. 

Exp. 100. Connect, in opposition, two cells of different kinds ; 
e.g., a Bunsen cell and a Daniell cell ; a deflection ensues, show- 
ing that the E.M.F. of the two cells is different, and that the 
E.M.F. of voltaic cells depends upon the material used. 

MEASUREMENT OE ELECTRO-MOTIVE EORCE. 

Exp. 101. Measure the E.M.F. of a voltaic cell as follows : 
Take, for example, a Bunsen cell, and connect in opposition 
with it a Daniell cell, and introduce into the circuit with this 
combination a galvanometer. There will be a deflection of the 
needle. Add other Daniell cells, one at a time, in series, in 
opposition to the Bunsen cell, until there is no deflection, or 



28 LABORATORY EXERCISES. 

only a slight deflection. Inasmuch as the E.M.F. of the 
Daniell cell is (about) 1 volt, the E.M.F. of the Bunsen cell 
will be approximately as many volts as the number of Daniell 
cells required to neutralize it. 

Exp. 102. Measure E.M.F. by method of equal deflections, 
the standard cell being E, the one to be compared being E'. 

Take the deflection of E and call it R ; then take the deflec- 
tion of E' and call it R\ adding resistance to make the deflec- 
tion the same. Then 

7?' 
E'=E~ 
R 

Exp. 103. Measure E.M.F. by comparison. Call the E.M.F. 
of two batteries E and E' ; join them up successively in circuit 
with the same galvanometer, and, by varying the resistance, 
cause them both to give the same deflection ; their forces will 
then be in direct proportion to the total resistances in circuit in 
each case ; 

7?' 
or, E< = Ex—, 

R 

when R (including that of battery, galvanometer, and the ad- 
justable resistance) represents the resistance with E, and R' 
with E'. 

Exp. 104. Place the cell whose E.M.F. is to be measured in 
circuit with a galvanometer ; it produces a deflection of d de- 
grees ; then add enough resistance r to reduce the deflection 
to d' degrees, say 10 degrees less than before. Now substitute 
the standard (Daniell) cell in the circuit, and adjust the resist- 
ances through a rheostat till the deflection is <K as before : then 
add enough resistance r' to reduce the resistance to d'. Now, 
calling E the E.M.F. of the battery to be measured, and E' the 
E.M.F. of the standard battery, 

r' :r::E': E, whence E = — , 
r 

since the resistances which will reduce the current equally will 

be proportional to the electro-motive force*. 



ELECTRICITY. 29 

MEASUREMENT OF INTERNAL RESISTANCE. 

Exp. 105. Measure the internal resistance of battery cells as 
follows. Take two cells which, connected in opposition as in 
Exp. 98, will give no current ; then introduce a third cell into 
the circuit with these two cells and a galvanometer, and a deflec- 
tion will be produced by the latter cell. Note the deflection, 
remove the pair of cells connected in opposition from the cir- 
cuit, and in its place introduce the rheostat, and measure the 
resistance of the pair of cells which has been removed. One- 
half of this resistance is the resistance of a single cell. 

Exp. 106. Put the battery in circuit with galvanometer, and 
note the deflection. Halve the tangent of the deflection by 
introducing resistance. The resistance introduced is equal to 
the original resistance — that of the battery and the galvanometer 
coil. Deduct from the resistance introduced the resistance of 
the galvanometer, and the remainder is the resistance of the 
battery. 



EFFECTS OF POLARIZATION. 

Exp. 107. Introduce a Bunsen cell, whose zinc is well amal- 
gamated, into circuit with the galvanometer, and watch from the 
commencement the deflection of the needle for about ten min- 
utes. The deflection will diminish somewhat during the first 
five minutes, and afterward remain quite constant. A deteriora- 
tion of current, greater or less, due to polarization during the 
first few minutes of its use, takes place with the best batteries, 
and proper corrections should be made for the same in all 
experiments in electrical measurements. 

Exp. 108. Connect in opposition a fresh cell with a cell which 
has been working from five to ten minutes, introducing a gal- 
vanometer into the circuit. A deflection of the needle shows 
that the E.M.F. of the fresh cell is greater than that of the 
other. 



30 LABORATORY EXERCISES. 

CURRENT THE SAME AT ALL POINTS OF A CIRCUIT. 

Exp. 109. Introduce into circuit a resistance of 52 ohms 
(about the resistance of four miles of telegraph wire) through a 
rheostat ; and. between the positive terminal and the rheostat, a 
galvanometer. Measure the current at this point, the beginning 
of its journey. Then insert the galvanometer between the 
rheostat and the negative terminal, and measure the current 
near the end of its journey. Finally introduce two rheostats 
into the circuit, and the galvanometer between them, and throw 
a resistance of 26 ohms from each into the circuit. Compare 
the results of the three trials, and determine whether they verify 
Law VI., page 68. 

EFFECTS OF DIFFERENT METHODS OF JOINING CELLS. 

Exp. 110. Take a single cell, and introduce into its circuit a 
resistance coil and galvanometer. By means of the coils throw 
a resistance of (say) 15 ohms into the circuit. Note the 
deflection. Introduce another similar cell into the circuit con- 
nected abreast (see § 183 of the Physics) with the first, still 
retaining in the circuit the 15 ohms resistance. The deflection 
is very slightly increased if at all, by the introduction of the 
additional cell. Find and compare the currents in the two 
cases, assuming that the E.M.F. of the (Bunsen) cell is 1.8 
volts, and r = 0.5 ohm, and R= 15 ohms, disregarding the re- 
sistance of the galvanometer. It will be seen, botli from the 
experiment and the calculation, that in this case almost no 
advantage is gained by using two cells connected abreast 
instead of one. By joining the two cells abreast, we virtually 
make one cell of double size, and thereby reduce the internal 
resistance one-half. But as the internal resistance of a single 
cell (0.5 ohm) is a small portion of the whole resistance in the 
circuit, the advantage gained is proportionately small. 

Exp. 111. Connect the two cells tandem, retaining the exter- 
nal resistance of 15 ohms in the circuit. The deflection is now 



ELECTRICITY. 31 

much greater than when a single cell is used in the circuit. 
Compare the currents in the two cases ; also, compute by Ohm's 
law the currents in the two cases. Here the resistance of the 
battery is double that of a single cell, or the whole resistance 
of the circuit is increased 0.5 ohm by the introduction of an 
additional cell. But this is a small portion of the entire 
resistance, and is, therefore, in this case, of little consequence. 
On the other hand, the E.M.F. of the battery is doubled. 
Consequently, the current, as shown both by experiment and 
calculation, is nearly doubled. 

Exp. 112. Introduce a galvanometer into a circuit with a 
single cell by means of short, thick copper wires, and note the 
deflection. Then connect two similar cells abreast, and note 
the deflection. The deflection is much increased by the intro- 
duction of the additional cell. Compute the current in each 
case, assuming that the internal resistance of each cell is 
0.5 ohm, and the external resistance is too small to be regarded. 
In both cases the whole resistance of the circuit is the internal 
resistance of the battery. By connecting two cells abreast, 
we reduce this resistance one-half, and consequently double the 
current, as shown both by experiment and calculation. 

Exp. 113. Connect the two cells used in the last experiment 
tandem, and note the deflection. The deflection is no greater 
(or very little greater) than that obtained with a single cell in 
circuit. Calculate the current in this case. By connecting 
the two cells tandem we double the E.M.F. of the battery. 
This of itself would double the current, but ou the other hand 
we double the resistance of the circuit. This would counter- 
balance the advantage gained by an increased E.M.F. 

Exp. 111. Introduce through a rheostat 1 ohms' resistance 
into a circuit with two Bunsen cells connected abreast. Note 
the deflection. Then connect the same cells abreast, and note 
the deflection. Compute the currents in both cases, and deter- 
mine whether the results of the experiments verify your cal- 
culations. 



32 LABORATORY EXERCISES. 

Exp. 115. Introduce through a rheostat a resistance of 2 
ohms into a circuit with six Bunsen cells connected tandem. 
Note the deflection. Remove from the circuit five of the cells, 
and obtain the deflection with a single cell in circuit. Connect 
the six cells abreast, and obtain the deflection. Finally, con- 
nect the six cells as a pair of triplets as follows. Take three 
cells and connect the three zincs with one another ; also the 
three carbons with one another. Connect the other three cells 
in the same manner. Then connect the zinc of one of the 
triplets with the carbon of the other triplet. In connecting the 
triplets with each other, it is immaterial at what point of the 
zinc combination or the carbon combination the connection is 
made. For example, the copper wire which is to connect the 
triplets may be attached to any one of the three zincs, or to the 
wire which connects the three zincs, for the three zincs in 
the triplet are to be regarded and treated in every respect as 
though it were one zinc. The same is true of the carbon trip- 
let. Compute the current in each of the four cases, assuming 
that the E.M.F. of each cell =1.7 volts, and r of each cell 
= 0.5 ohm, the external resistance in each case being (disre- 
garding the resistance of the galvanometer) 2 ohms. Observe 
whether the results obtained by calculation are verified approx- 
imately by the results obtained by experiment. 

Exp. 116. Introduce a voltameter or apparatus for decompos- 
ing water into a circuit, and measure its resistance, and deter- 
mine in what way a battery of two cells (Bunsen) should be 
connected for electrolytic purposes. 

ELECTRO-MAGNETS OF VARYING RESISTANCE. 

Exp. 117. Introduce into a circuit, with a single (Bunsen) 
cell, an electro-magnet wound with very fine wire. Measure the 
resistance of the electro-magnet ; then introduce by means of 
the rheostat an additional resistance equal to that of the magnet. 
Note the deflection, and test the strength of the electro-magnet 



ELECTRICITY. 33 

by applying an armature to its poles, and observing the force 
necessary to pull it off. Then substitute for this electro-magnet 
an electro-magnet wound with coarse wire, allowing the same 
resistance through the rheostat to remain in circuit. Test the 
strength of the electro-magnet as before. 

Exp. 118. Introduce the electro-magnet of fine wire into cir- 
cuit, without any other resistance except that of the battery. 
Test the strength of the magnet as in the last experiment. 
Then for this substitute the magnet of coarse wire, and test the 
strength of this magnet. Do the results of the last two experi- 
ments verify the first law of electro-magnets, page 72? 

Exp. 119. Place in circuit the same magnet of fine wire as 
used in the last experiment, with an equal resistance through 
the rheostat. Introduce an additional cell connected in series 
with the first. Test the strength of the electro-magnet. Note 
the deflection produced in the galvanometer introduced into the 
circuit. Compare the present current with the current when a 
single cell was in circuit with the same resistances. 

Exp. 120. Place in circuit with a Bunsen cell a telegraph 
key, and a sounder of low (about 3 to 9 olnns) resistance 
(such as furnished by the Author) . The cell will work the 
sounder strongly. Then introduce into the same circuit with 
the last a rela}- of about 25 ohms resistance (such as furnished 
by the Author) . The siugle cell will probably not work either 
the relay or sounder, in consequence of the increased resistance. 
If either works it will be the relay. 

Exp. 121. Now introduce two cells connected tandem into 
circuit with the relay and sounder. The relay will now work, 
but the sounder probablv will not ; or, if the sounder does work, 
it will be only feebly. In case it does work, introduce sufficient 
additional resistance into the circuit through a rheostat, so to 
enfeeble the current that the sounder will not work, but not suf- 
ficient to prevent the relay from working. Why will the relay 
work and not the sounder? 

Exp. 122. Take a third voltaic cell, and construct a local cir- 



34 LABORATORY EXERCISES. 

cuit, with the sounder in this circuit, inserting the wires of this 
circuit in the two screw cups of the relay that have not so far 
been used. Now manipulate the key in the line circuit, and this 
will work the relay in the same circuit ; the relay working will 
open and close the local circuit, causing the sounder in this 
circuit to work. 

Exp. 123. Introduce, through a rheostat, into the circuit with 
the relay a resistance (say) of 65 ohms (which is about the 
resistance of five miles of telegraph wire) . Then introduce a 
sufficient number of cells, connected in series, to work the relay. 
Compute the current which works the relay, first measuriug the 
resistance of the relay, and assuming that the r of each cell = 0.5 
ohm, and that the E.M.F. of each cell =1.8 volts. 



DIVIDED CIRCUITS. 

Exp. 124. Introduce into a circuit with a single cell B (Fig. 
10) a galvanometer 6r, and rheostat i?, and through the last a 
resistance of 4 ohms. Note the deflection. 
Now take a short copper wire and attach it to 
the two batter} 7 wires at any two points, as a 
yl and b, by winding several turns around the 
wire so as to make good connections. If 
double connectors are used, these connections 
may be made easily and quickly. Note the 
deflection, and observe that it is very much less than before. 
The wire which serves as a bridge between the two battery 
wires is called a shunt. Next cut the shunt wire at some point, 
as at 6r', and introduce the galvanometer at that point. Close 
the circuit at the point from which the galvanometer was re- 
moved, by twisting the wires together, or, better, by inserting a 
double connector. A deflection of the galvanometer shows that 
a current is passing through the shunt wire. Observe that the 
deflection is greater than it was when the galvanometer was in 
circuit with the rheostat. We thus learn that the current be- 




ELECTRICITY. 35 

comes divided unequally at one of the two points a or b (ac- 
cording to the direction in which the current is flowing) , and 
the two portions flow through the two paths open to it from 
that point, the larger portion flowing through the smaller resist- 
ance. Compare the currents in the two branches and see if the 
results of the experiment verify the law of divided circuits, 
page 69. 

Exp. 125. Remove the galvanometer from the shunt wire, 
closing the circuit at that point, and introduce it into the circuit 
between the battery and point a or b. Observe that the deflec- 
tion is now larger than it was before the circuit was shunted, 
as we might expect when we reflect that by introducing the 
shunt wire we virtually increase the size of the wire in the 
circuit beyond the points a and b, and thereby reduce the resist- 
ance of the circuit. 

If a main wire is split or divided into many branches, all of 
equal resistance, the current will with absolute certainty and 
perfect exactness divide itself equally among all the branches. 
This fact renders the distribution of an electric current among 
any number of electric lamps, so that all shall be equally 
lighted, perfectly simple and easy, even more easy than the 
equal distribution of gas among a number of burners. If, 
however, it is desirable to make an unequal distribution, it may 
be very easily done by suitably varying the resistance in the 
different branches. 



THREE METHODS OF TRANSFER OF ELECTRIFICATION. 

Exp. 126. Charge a Ley den jar in the usual manner; place 
one ball of a discharger on the outside coating, and bring the 
other ball on a level with and about 8 cm from the ball of the jar ; 
suspend between the last two balls by a silk thread a pith ball 
covered with tin or gold foil. The pith ball acted on by the 
electric force will vibrate between the two balls, carrying elec- 
tricity from one to the other, and thus gradually discharge the 



36 



LABORATORY EXERCISES. 



jar. A transfer or current of electricity is thus kept up between 
the two balls in virtue of the motion (caused by the E.M.F.) 
of the electrified body which conveys the electricity as it moves 
from place to place. This phenomenon may be called a cur nut 
of convection. 

"Electricity carried through space on a charged body has 
exactly the same magnetic effect on a stationary magnetic needle 
as if it had been conducted." — Rowland. 

Exp. 127. Charge the jar again and establish an electrical 
connection between the ball of the jar and the outside coating 
through a discharger. There will be a transference of elec- 
tricity through the wire of the discharger, but the wire itself 
does not move. What takes place in the wire is called a 
current of conduction. 

When a compound body is decomposed by an electric current, 
the mode in which the current is transmitted through the elec- 
trolyte is called electrolytic conduction, and is always associated 
with a flow of the components of the electrolyte in opposite 
directions. 



EQUIPOTENTIAL LINES AND LINES OF FLOW. 

Exp. 128. Take a sheet of tin foil AB about 25 cm square, 
spread out over a sheet of paper ; at points x and t apply the 
two terminals of a battery. Then 
with two platinum electrodes con- 
nected with an astatic galvanom- 
eter, feel around for points c, c r , 
c", etc. (Fig. 11), such that when 
the platinum terminals are ap- 
plied at any two of them the 
galvanometer shows no deflec- 
tion. Prick through these points 
into the paper. 

Points whose potentials are equal are called equipotential 
points. A line connecting a series of such points, as cc'", is called 





jfc^S. 






'£^7 — 1— — 









Fig. 11. 



ELECTRICITY. 37 

an equipotential line. As the experiment shows and the term 
implies, there is no cm-rent along an equipotential line. 

If the direction of the E.M.F. at different points in the field 
are found (as may be done by suspending a very delicate 
elongated conductor successively oyer these points, the axis of 
the conductor placing itself in the direction of the force at 
each point), and if a Hue be drawn so that its direction at 
every point coincides with the direction of the electric force 
at that point, such a line is called a line of force. It is found 
that the direction of the E.M.F. is everywhere perpendicular to 
the equipotential lines, and hence the line of force everywhere 
cuts these lines at right angles and terminates in the electrified 
points or centers of force as x and t. By drawing a number of 
such lines as in Fig. 11, the direction of the force at different 
parts of the field may be indicated. Since these hues also 
indicate the direction in which the transfer of electrification 
takes place, they are called lines ofjfoic. 

; ' If from a sheet of indefinite extent we cut off a portion 
bounded by hues of flow, we shall not affect the electrical dis- 
tribution, i.e., the forms of the equipotential hues will be the 
same as before the sheet was cut." See Gordon's ; ' Electricity 
and Magnetism," pp. 31-41, Vol. II. 

IXDTUCTIOX COILS. 

Exp. 129. Connect the secondary coil of an induction coil 
with a (tangent) galvanometer (see Fig. 170, Physics), and the 
primary coil with a battery. Introduce the latter coil into the 
former, and as soon as the needle becomes quiet, withdraw it, 
noting the deflections. If the deflections are too large, so as to 
throw the needle beyond 90°, the deflections may be reduced any 
desired amount by means of a shunt of suitable resistance. 
Then, allowing the primary coil to remain within the secondary 
coil, plunge an iron core within the former, and soon withdraw 
it, noting both deflections. Finally, introduce both primary coil 



38 LABORATORY EXERCISES. 

and its core into the secondary coil, and presently withdraw it, 
noting the deflections. It will be seen that the core is a very 
important adjunct to an induction coil. 

Exp. 130. Remove the core from the induction coil ; let the 
primary coil remain within the secondary coil, and introduce the 
former together with the vibratory circuit-breaker into circuit 
with a single voltaic cell. Connect a pair of handles with the 
terminals of the secondary coil, and let a person grasp these 
handles, holding one in each hand. Let another person gradu- 
ally introduce a core into the primary coil. The intensity of 
the shock will rapidly increase until it will become unendurable. 
The intensity of the shock can be governed by the distance the 
core is inserted in the coil. If the coil without the core should 
give painful shocks, the primary circuit may be shunted through 
a rheostat and any desirable resistance. 

Exp. 131. Connect, by means of flexible wires several meters 
long, the secondary coil of a small induction coil with an astatic 
galvanometer, and turn it end for end in an E. and W. plane, 
and feeble currents will be induced therein through the influence 
of the earth's magnetism. 

Exp. 132. Connect a telephone receiver with a sensitive long- 
coil galvanometer, and produce induced currents by pressing the 
disk of the telephone inward with the finger and allowing it to 
spring back. 

Exp. 133. Shake the hand rapidly with the fingers spread out 
in front of a Geissler tube illuminated by an induction coil, and 
the light of the spark, being intermittent, will produce a large 
number of images. 

Exp. 134. Select a Geissler tube contracted along the centre 
so that the light is reduced to a narrow, bright line. Place the 
tube in a vertical position, and a bi-sulphide of carbon prism a 
little distance from it, and project a spectrum of the light upon 
the screen. The spectrum thus obtained consists of several 
bright lines characteristic of the gases which they contain. 

Exp. 135. Insulate a Ley den jar, and connect the secondary 



ELECTRICITY. 39 

terminals of a Ruhnikorff coil with the two coatings of the jar. 
Lead wires of different metals from these coatings so that their 
extremities may be from one to two inches apart, and observe 
the character of the spectra produced during the discharges. 
The spectra produced should be characteristic of the metals 
used. 

Exp. 136. "To show the production of induced currents in a 
telephone and then* physiological effect. Attach the ends of 
the wires from a Bell telephone to the leg muscles of a frog, and 
speak in the telephone. The pronunciation of the word ' sucker' 
causes the leg to move or 'jump/ while 'He still' has scarcely 
a sensible effect." 

Exp. 137. Take a coil of wire of considerable size, e.g., the 
primary coil of an induction coil (Fig. 170, Physics), and a soft 
iron core a little longer than the coil and about one -half inch in 
diameter, attach to it the poles of a battery of two cells of 
Bunsen, so connected as to give a strong current. Make and 
break the circuit by touching and separating the extremities of 
the wires or by a telegraph key inserted in the circuit, — the key 
and coil may well be in separate rooms, — and a person placing 
his ear near the core will hear distinct clicks ("magnetic ticks") 
every time the circuit is broken and closed. These sounds pro- 
ceed from molecular disturbances attending magnetization and 
demagnetization. Experiments have determined also, that iron 
rods are elongated on being magnetized., and retract on losing mag- 
netism, and that heat is developed in the iron by both processes. 

Exp. 138. Lead the secondary terminals of a Ruhmkorff coil 
to the opposite coatings of a Ley den jar (the larger the better) , 
and place one ball of a discharger on the outer coating and hold 
the other ball near the ball of the jar. The discharges will be 
less frequent as the jar has to be charged between each spark, 
but the sparks will be much more brilliant and the reports almost 
deafening. The jar should be placed on an insulating stand. 

Instead of a single jar, several connected "in cascade" (see 
p. 44) may be used. All the jars in the cascade must be insulated. 



40 LABORATORY EXERCISES. 

EXTRA CURRENTS. 

Exp. 139. Connect one of the poles of a battery with one 
extremity of a file as in Fig. 132 of the Physics, and draw the 
other pole over its roughened surface. Yellow sparks fly from 
the file. These are incandescent particles of iron projected 
into the air, caused by the fusion of the metals at the points of 
contact. Introduce into the circuit a helix and repeat the 
experiment. Now, in addition to the yellow sparks which fly 
from the file, there will be seen much brighter and whiter sparks 
following the electrode as it passes over the surface of the file. 
These sparks are confined to the surface of the file, and are 
caused by the passage of the extra currents generated at each 
break of the circuit (the extra current on "making" gives no 
spark) . These sparks are of the same nature as those which 
pass between the secondary poles of an induction coil. Finally, 
insert an iron core in the helix and again repeat the experi- 
ment. The sparks produced by the extra currents are still 
brighter. 

ELECTROLYSIS. 

Exp. 140. Lay upon a metallic surface — e.g., a strip of brass, 
copper, or tin — a piece of white paper moistened with a solution 
of salt, to which a few drops of ferrocyanide of potassium have 
been added ; connect the negative electrode of a battery with 
the metal strip ; make a positive electrode of iron by attaching 
to the copper wire a short piece of iron wire ; move the free end 
of the iron wire over the paper, as if to write on it, and a blue 
mark will be made on the paper. See § 243, Physics. 

Exp. 141. Dissolve about l g of iodide of potassium in 20 cc of 
water ; make a starch paste by boiling a small quantity in a tea- 
spoonful of water ; take as much of the paste as will lie upon a 
quarter of an inch of the point of a penknife blade, and stir it 
in the solution. Moisten a piece of white paper with this solu- 
tion ; lay it upon a metallic surface ; connect one of the copper 



ELECTRICITY. 41 

poles of the battery with the metallic surface, and, with the 
other pole, write upon the paper as with a pencil. 

HEATING EFFECT. 

Exp. 142. Provide a glass flask of one liter capacity. Pass 
water-tight through a cork two No. 16 Kerite covered copper 
wires, and, close to the cork, attach to one an amalgamated 
zinc rod, and to the other a carbon rod, the rods being of 
sufficient length to reach to about half the depth of the flask. 
Connect the two extremities of the wire, outside the flask, by 
a platinum wire about 2 cm long and size No. 32. Introduce into 
the flask bichromate of potash solution enough to nearly reach 
the lower extremities of the rods when the flask is upright. 
When you would show the heating power of electricity, light 
gas by means of the heated wire, etc., you have only to invert 
the flask. 

THERMO-ELECTRIC CURRENTS. 

Exp. 143. Take a strip of sheet copper about 15 inches long 
and three-fourths of an inch wide, and a strip of zinc of the 
same dimensions. Lay them one upon the other, and fold over 
each end upon itself for about half an inch, and hammer the 
joints flat, so that they shall 
hold together quite firmly. 
Then separate the two strips 
into a somewhat elliptical or 
rectangular shape, as shown in 
Fig. 12. Cut a hole through 
the center of one of the strips, p. n 

and pass the wire support of a 

magnetic needle through it. Place the band in the magnetic 
meridian parallel with the needle. Direct a flame against one 
of the junctions, and note the deflection, and determine the 
direction in which the current traverses the band, i.e., whether 




42 LABORATORY EXERCISES. 

the current passes from the heated junction through the copper 
or the zinc strip. 

Exp. 144. Attach to a (astatic) galvanometer a copper and 
an iron wire. It is well to let the wires rest upon pieces of ice 
near the galvanometer. Join the free extremities of the wires, 
and apply a Bunsen or spirit flame at the junction. A cur- 
rent is established, and the E.M.F. continues to increase till 
the temperature of the junction is about 284° to 300° C. ; then, 
on raising the temperature higher, it begins to decrease, and, 
finally, is reversed, as is shown by a reversal of the deflection. 

MAGNETISM. 

Exp. 145. Heat red-hot an iron ball, and suspend it by a brass 
or copper wire. Bring a pole of a powerful electro-magnet near 
to the ball, and the latter will not be attracted. Keeping the 
magnet in the same position, wait until the ball has cooled, and, 
when sufficiently reduced in temperature, it will be attracted. 
This shows that iron cannot be magnetized when red-hot. 

Exp. 146. Hold a soft- iron rod about l m long in the same 
position as that taken by a dipping-needle. Suspend a small 
magnetic needle, and bring it near the end of the poker which 
points downwards. The N. pole of the needle will be repelled, 
and its S. pole attracted, showing that this end of the poker is 
a N. pole. Pass the needle upwards close to the poker, and its 
8. pole will continue to be attracted until the middle is reached, 
after passing which the needle will turn round, and its N. pole 
will be attracted ; and so onwards to the upper end of the poker, 
which we say has been magnetized by the inductive action of the 
earth. 

Exp. 147. Effect of percussion. While in the position men- 
tioned in the last experiment, strike the end of the poker sev- 
eral times with a hammer. This percussion will be found to 
have conferred coercive force upon the poker, which now retains 
its magnetism in any position. 



ELECTRICITY. 43 

Exp. 148. The earth's magnetic force directive and not trans- 
lative. Float a, small magnetic needle, fixed horizontally to 
a cork, on water. The needle will point nearly N. and S., 
but will not float towards the north. This is owing to the great 
distance of the earth's magnetic poles, which are both attracting 
and repelling the needle. The attraction and repulsion being 
practically equal, there results only a rotation, and no motion 
of translation. The two forces act as a mechanical couple. 



EXPERIMENTS WITH LEYDEN JARS. 

Exp. 149. Charge the jar by grasping the outer coating with 
the hand, and holding the ball about half an inch from the 
prime conductor. When the sparks cease to pass from the 
conductor to the ball, the jar is charged. 

Exp. 150. Place a charged jar on an insulating support, and 
touch with a finger the ball, keeping the other hand away from 
the outer coating. A faint spark will pass from the ball to the 
finger. Now touch the outer coating, and a faint spark will 
pass from this coating to the finger. Continue touching alter- 
nately the ball and outer coating until the jar is discharged, or 
as long as a spark can be obtained from either ball or coating. 

Exp. 151. Place a Leyden jar on an insulated support, and 
bring the ball near the prime conductor. Only a few sparks 
will pass from the conductor to the ball. After turning the 
machine for a time, discharge the jar, when it will be found 
that it contained only a slight charge, thus showing that an 
insulated jar cannot be heavily charged in the usual manner. 

Exp. 152. Take a strip of leather one-half inch wide and long 
enough to encircle the jar. Through this strip, at intervals of 
one inch apart, thrust sharp pointed tacks. Bind the strip 
around the jar and over its outer coating, so that heads of the 
tacks will press against the jar. Now place the jar upon an 
insulating support, and it will be found that it can be easily 
and heavily charged. 



44 LABORATORY EXERCISES. 

Exp. 153. Take three or four Leyden jars, and place each 
horizontally on its side, on separate insulating supports. Bring 
the ball on one of the jars near the conductor of the machine, 
and the ball of a second jar near the outer coating of the first 
jar, and so on for the third and fourth jars. Let the outer 
coating of the last jar be held in the hand or connected with the 
earth, or, still better, connected by a chain with the negative 
conductor of the machine. Now, as the machine is worked, 
each jar will become charged. The interior coating of each jar 
will be charged with a different kind of electricity from that of 
the jar which precedes it, and will be less heavily charged than 
its predecessor. Each jar may be separately discharged in the 
usual manner, or the whole may be simultaneously discharged 
by first connecting all interior coatings with one another, and 
all the exterior coatings with one another ; then, with a dis- 
charger, discharge the whole as if it were a single jar. 

Exp. 154. Charge a jar from the prime conductor of an elec- 
trical machine, and, rubbing the ball over the resinous surface of 
an electrophorus, or a plate of glass covered with shellac, draw 
a figure upon its surface. Charge another jar from the negative 
conductor of the machine, and draw another figure across the 
last figure. Now sift the dust of a mixed powder composed of 
red lead and sulphur over this surface. The two powders will 
separate and arrange themselves in beautiful radiations, the red 
lead along the lines formed by the negative jar, and the sulphur 
along the lines formed by the positive jar. 

Exp. 155. Charge separately, and in the same manner, several 
Leyden jars. Place them in a row on insulating stands, and 
connect them in series, after the manner of a voltaic battery 
whose cells are connected tandem, i.e., connect the inner coat- 
ing of the one with the outer coating of the next throughout the 
series. Then discharge the whole, by connecting the outside 
coating of the jar at one end of the row with the inside coating 
of the jar at the other end of the row. Very long and intense 
sparks are thus obtained. 



ELECTRICITY. 45 

MISCELLANEOUS EXPERIMENTS. 

Exp. 156. Take 20 threads of linen each 12 inches long, and, 
placing them together and parallel with one another, so as to 
form a bundle, tie strings around the bundle 1 inch from either 
end. Suspend the bundle, by means of a short wire, from the 
prime conductor of the machine. As the machine is worked 
the threads will recede from each other and form a balloon- 
shaped figure. 

Exp. 157. Let some ingenious boy cut from a cork or pitch a 
figure of the body of a spider, attaching to it linen threads 
about 1^ inches long for legs. Suspend the spider from some 
support by means of a silk tlnead. Charge two Leyden jars, 
one with negative and the other with positive electricity. Place 
the two jars near each other on a table and bring the spider 
between the two balls. It will be first attracted to one of the 
balls, then repelled by it and attracted by the other ball, and 
will continue to vibrate for a considerable time between the two 
balls, the legs at each end of the swing grasping the ball as if to 
support itself. At each swing it carries a small charge of elec- 
tricity from jar to jar, and thereby gradually discharges the two 
jars. 

Exp. 158. Take a warm, dry, wide-mouthed, shallow bell- 
glass or a glass fruit-dish, and rub the ball (better a point) con- 
nected with the prime conductor, while the machine is in opera- 
tion, over the interior surface of the glass vessel until the 
interior surface becomes highly excited ; then place it so as to 
cover a handful of pith balls lying upon a table. An animated 
scene will follow. 

Exp. 159. Insulate an electrical machine, and connect the 
positive and negative conductors by a wire. However rapidly 
the machine is worked no charge will accumulate, thus showing 
that equal quantities of -f and — electricity are produced. 

Exp. 160. Steep Swedish filter paper in a mixture of equal 
volumes of nitric and sulphuric acids, wash in abundance of 



46 LABORATORY EXERCISES. 

water, and dry. Lay it on a sheet of waxed paper and rub 
briskly with flannel or silk, and use as an electrophorus. 



METAL SCREENS. 

Exp. 161. Electrify a glass or sealing-wax rod and interpose 
between it and a pith ball a metal screen (e.#., a strip of wire 
gauze) connected with the earth ; no induction takes place. In 
general, a body may be protected from inductive influence by 
covering it with a meted or wire gauze cage connected to earth. 

' ' The problem of lightning protection is to produce a space 
into which electricity cannot enter. If a hollow shell of copper 
were made, for example, a person inside would be perfectly 
safe, though all the lightning of the heavens were playing about 
him, because the electricity would pass around the outside, 
which would be of superior conductivity, instead of leaping 
across the space within. So, if a house were enclosed in a 
cage of copper, the lightning would pass around the cage instead 
of through it. The best method of protecting a house, there- 
fore, would be to erect a central rod on the roof, from which 
conductors would pass to the four corners and then down to the 
ground. But the rods must not stop there ; they must continue 
beneath the house, surrounding it below as above, completely 
enclosing the bottom as well as the roof. If it were desired to 
make this cage more complete, conductors might be carried 
from the central rod down the four sides of the house, as well 
as down the corners." — Rowland. 



SOUND. 47 



SOUND. 

VIBRATIONS OF BARS. 

Exp. 162. Let a carpenter prepare a pine bar as nearly uniform 
throughout as possible and 5 feet long, 1 inch wide, and -^ inch 
thick. Cut it, as nearly as possible, into the following lengths, 
viz. : 3, 3.8, 4.7, 5.3, 6.5, 8.3, 10.5, and 12 inches. Arrange 
theru side by side on a table, a convenient distance apart ; com- 
mencing with the longest, raise and let drop upon the table suc- 
cessively each bar, and determine by the pitch of the noise 
(every noise has pitch) produced on striking the table the rela- 
tion between the vibration period of bars and their lengths. 

Exp. 163. Provide three pine bars, each 4 inches long and 
1 inch wide, and respectively ^-, 3^-, and f inch thick. Drop 
them, and determine the relation between the vibration period 
of bars and their thickness. 

VIBRATIONS OF BARS FIXED AT ONE END. 

Exp. 164. Take a lath about l m long and 3 to 5 cm wide, and 
about 5 mm thick, lay it upon a table with about two- thirds its 
length projecting beyond the table, press it firmly upon the table 
with one hand, about 6 cm from the edge of the table, and with 
the other hand bend the free part upward and let go. Notice 
the pitch of the vibration. Then shorten the projecting part 
one-fourth, one-half, three-fourths, etc., at a time, and notice 
the change in pitch. 

LONGITUDINAL VIBRATIONS. 

Exp. 165. Grasp between the thumb and fore-finger of one 
hand a glass tube (about l m long and 6 mm in diameter) about 
one-third its length from one end. Place on the palm of the 



48 LABORATORY EXERCISES. 

other hand a damp cotton cloth, and grasp the tube just below 
the thumb and finger and quickly draw it along the tube (the 
longer part) to the end. Repeat this movement several times 
until you obtain loud, shrill sounds as the result of longitudinal 
vibrations. 

Exp. 166. Take a tube half the length of the last and pro- 
duce longitudinal vibrations. The time of a complete vibration 
being that required for the sonorous pulse to run twice to and 
fro over the tube, how should the pitch of the latter compare 
with that of the former ? 

Exp. 167. Grasp in a vice, in a nearly horizontal position, a 
steel rod of about the same dimensions as the first glass rod, 
and about one decimeter from one end. Sprinkle powdered rosin 
on a leather glove, e.g., a dogskin or kid glove, and with the 
rosined fingers excite vibrations by friction lengthwise the rod. 
Suspend by a string a small ivory ball so that it may touch the 
end of the rod nearest the vice. The ball will bound back and 
forth against the rod when it is excited, thus showing that the 
vibrations are longitudinal. 

Exp. 168. Ascertain the wave-length in accordance with the 
above in wires of different substances by throwing them into 
longitudinal vibrations, and bringing them into unison (by vary- 
ing the length) with that of a tuning-fork whose vibration rate 
is known. Then calculate the velocity of sound in each sub- 
stance. 

DIAPASONS. 

Exp. 169. Take a diapason (a large tuning-fork), and by means 
of sealing-wax attach to the end of one of its prongs a narrow, 
thin piece of sheet brass, cut at one end to a fine point, or a 
very fine (No. 30) wire. Prepare a smoked glass by passing 
one of its surfaces over a candle flame. Set the fork in vibra- 
tion, and draw the style over the smoked surface with uniform 
velocity, allowing the style lightly to touch the glass. Next draw 
a bow rapidly and with slight pressure at a point about two- 



SOUND. 49 

thirds the distance from the end of the prong to the handle, and 
again draw the style across the smoked glass. This should give 
a graphical representation of the harmonic of the fundamental 
of the fork. If you do not succeed in this, bow the fork as 
before, and lightly touch with the extremity of the finger-nail a 
point about one-third the length of the prong from its extremity. 

The sound of a tuning-fork, when set in vibration in the usual 
manner, contains, beside the fundamental, numerous overtones ; 
but the interval between them and the fundamental is very great. 
Consequently, the overtones are very evanescent, and soon leave 
the fundamental practically pure. This important acoustical 
property is very much increased when the stem is applied to a 
table or resonance-box, which reenforces the fundamental at the 
expense of the others. 

Exp. 170. Produce the fundamental note, and, while it is 
sounding, draw the bow so as to give the harmonic, and imme- 
diately apply the style to the smoked glass. A curve should 
thus be obtained, resulting from the two systems of vibrations, 
consisting of smaller and shorter wavers superimposed upon 
larger and longer ones. If you succeed in obtaining good re- 
sults, and wish to preserve them, first hold the blackened sur- 
face in the vapor of boiling alcohol, to remove the grease. 
Then pom- amber varnish over it, as when varnishing a photo- 
graphic negative, and allow it to dry. 

Exp. 171. Take a glass plate of the same size as a stereopticon 
slide. Blacken it as in the foregoing experiments, but do not 
apply a very thick deposit of soot, so as to make it quite opaque. 
Draw the style of the fork when not vibrating lengthwise the 
blackened surface, and about one-third its width from the edge. 
A simple straight line will be the result. Produce the funda- 
mental, and draw the style across the plate parallel with the 
line previously drawn. Produce the harmonic alone if possible, 
and draw the style across the plate parallel with the last two 
lines. Finally, produce the fundamental and the harmonic to- 
gether, and once more draw the style across the plate. Yarnish 



50 LABORATORY EXERCISES. 

the plate, and preserve it for use in projecting with the porte- 
lumiere graphical representations of actual vibrations. 

Exp. 172. Set a tuning-fork in vibration, and touch that string 
of a violin which is nearest its own pitch, and move it along 
the string to or from the bridge until a length of string is 
obtained which will vibrate in sympathy with the fork, when 
a loud sound will be given forth by the string. 



SAVART'S BELL, CHLADXI'S PLATE, AXD IXTERFERENCE- 
TUBE. 

Exp. 173. Set in vibration, by bowing, a Savart's bell (see 
catalogue of apparatus to illustrate sound), and notice the 
wavy sound produced as the result of interference of the 
fundamental with its overtones. The fundamental is pro- 
duced by the division of the edge into four segments. 

Exp. 174. By repeated trials determine the fundamental pitch 
of the bell. Produce the fundamental, and, at 90° from the 
point bowed, hold as near the edge of the bell as convenient 
without touching, one end of the open resonance-tube. The 
sound is strongly reenforced. Place the plunger in the remote 
end so as to convert it into a closed tube, preserving the same 
distance from the bell, and repeat ; the sound is only feebly 
reenforced. Gradually move the plunger inward until the 
maximum reenforcement is obtained, and determine what part 
the length of an open tube a closed tube should be to reenforce 
a sound of a given pitch. 

Exp. 175. AYith the same bell produce a sound of higher pitch 
than the fundamental, and, with the plunger, adapt the length 
of the closed tube to the sound. 

Exp. 176. Produce the fundamental, and reenforce with the 
closed tube its lowest overtone. 

Exp. 177. Remove the bell from its iron support, and mount 
in its place the Chladni's plate. Scatter evenly over its sur- 
face fine writing-sand from a wooden sand-box, such as fur- 



S0T7XD. 51 

nished by stationers. Bow the center of one edge with a rosined 
bow. producing the fundamental of the plate, known by the sand 
arranging itself in diagonal lines, so as to divide the plate into 
four segments. Note that in whatever way the plate is divided, 
there is always an even number of segments formed. 

Exp. 178. (a) Bow the plate near one of its corners, (b) Bow 
the plate in the middle of the edge next you, and, with the 
thumb and one finger of the left hand, damp the left edge at 
points one -fourth the length of the edge from each of its ex- 
tremities, (c) Obtain a variety of figures by varying the points 
bowed and the points damped. Make drawings of each, note 
the pitch of each, and describe the method by which each was 
produced. 

Exp. 179. Produce the fundamental by bowing the center of 
one edge, hold the orifices of the two prongs of the interference- 
tube over the opposite segments, and adjust the length* of the 
tube so as to strongly reenforce the sound. Then place the 
orifices over adjacent segments, and at the same distance from 
the plate as before. Interference causes a destruction of sound. 

Exp. 180. Produce " hydrogen tones " by introducing a flame 
of illuminating gas (at the narrow orifice of a glass tube) about 
2.5 cm long into a glass tube about 0.8 m long and S^ bore. 
At a little distance from the flame rotate the mirror furnished 
for manometric-flame experiments. Obtain tones of different 
pitch, and note the appearance of the vibrating flame in the 
rotating mirror. 

Exp. 181. Procure a tin flageolet at a toy shop. Over the 
mouth of the instrument slip a rubber tube, and connect the other 
end of the tube with a gas-tube nipple or other source of con- 
densed air or gas of any kind. Obtain a glass tube from 10 to 
15 ft. long, with a bore large enough to receive nearly the 
whole of the tapering flute. Turn on the gas slowly ; the first 
sound heard will probably be the lowest sound produced by the 
flute reenforced by the glass tube ; on turning on the gas with 
greater force, higher notes break forth one after another. As 



52 LABORATORY EXERCISES. 

many as twenty distinct notes have been produced in this way. 
If precipitated silica is scattered along the bore of the tube, it 
will collect at the nodes of the tube corresponding to each 
tone. 

Exp. 182. Allow the corner of a card to tap the edges of the 
siren plate as it is rotated, and you obtain all the phenomena 
derivable from the expensive Savart's wheel. 

For other experiments in the phenomena of sound, see the 
admirable little volume on Sound in the Experimental Science 
Series, by Prof . A. M. Mayer. 



53 



LIGHT. 
RADIOMETER. 

Exp. 183. Light a match, and hold the flame three or four 
inches from a radiometer. 

Exp. 184. Place a radiometer three or four feet from a gas- 
flame. 

Exp. 185. Hold the palm of your hand about an inch from a 
radiometer. 

Exp. 186. Find the greatest distance from lights of different 
kinds, e.g., candle, kerosene, spirit, and Bunsen (both the light- 
giving and colorless) flames, and the electric light, that motion 
in the radiometer can be produced, and thus compare the 
mechanical power of the ether waves proceeding from the differ' 
ent sources. 

DAYLIGHT PHOTOMETRY. 

Exp. 187. (Pickering.) Provide a box about 6 ft. long, 1 ft. 
wide, and \\ ft. deep. A wooden frame covered with black paper 
will answer the purpose. Cut a circular hole about 4 in. in 
diameter in one end, and cover it with blue glazed paper with 
the white side out. Let drop a drop of melted sperm-candle 
wax upon the center of the disk, rubbing it around with the 
finger so as to cover a circular space of about the size of a sil- 
ver dollar. Take a lath a little longer than the box, and about 
two inches wide, and cut a small hole in the end of the box 
containing the paper disk large enough to allow the lath to pass 
through it into the box, leaving one end projecting from the box. 
Upon the end of the lath in the box mount a lighted candle. 
The box should be properly ventilated by holes, so that the can- 
dle may not become extinguished. Taking hold of the exposed 
end of the lath, push the candle so far away from the paper disk 



54 LABORATORY EXERCISES. 

that, in a room where the intensity of light is to be measured, the 
paper disk will appear dark in its center. Then draw the candle 
slowly forward until the center appears neither darker nor brighter 
than the center of the disk. Measure the distance of the candle 
from the disk. (If the lath has a scale of inches marked off on 
it, the distance will be very easily ascertained by observing the 
portion of the lath extending from the box, and deducting that 
from the length of the lath.) Unity divided by the square of 
this distance gives a measure of the comparative brightness of 
the daylight under various circumstances. Carry the box into 
different parts of the same room, and into different rooms, and 
measure the intensity of the light. 

Exp. 188. With the daylight photometer measure the fading 
of the light at twilight. Other interesting experiments may be 
made with the same apparatus, by observing the intensity of 
light during an eclipse, and by comparing moonlight, and light 
of the Aurora, with daylight. 

MEASUREMENT OF REFRACTION. 

Exp. 189. Take "a tank with platinum electrodes," such as 
furnished by the author for projecting " phenomena attending 
electrolysis." Pour water into the tank, leaving about a quarter 
of an inch of electrodes exposed. Prepare two paper scales long 
enough to extend across the glass between the brass frame- 
work, and 4 mm wide, making the divisions of the scale 2 mm 
each. With flour paste apply one of these scales to the external 
surface of one of the glass sides, parallel with and about l cm 
below the surface of the water. Apply the other scale above 
and parallel with this, and so that the lower edge of the paper 
will be on a level with the upper extremities of the platinum 
electrodes. Place the eye about on a level with the surface of 
the water, so that it can, without moving, see both scales. If 
the eye is placed directly opposite one of the electrodes, it will 
appear at the same point on both scales ; but the other elec- 
trode will appear at different points on the two scales, and the 



LIGHT. 55 

difference will be the amount of refraction. Move the eye to 
different positions, and observe the amount of refraction in 
each. It will be still better to set up vertical wires outside the 
tank and view them through the liquid. 

LENSES. 

The lens accompanying the porte-lumiere is admirably adapted 
to the following experiments. 

Exp. 190. To find the focal length of a convex lens. Hold a 
convex lens in the sunlight, its face at right angles to the sun's 
rays. Behind the lens and parallel with it hold a piece of light 
brown paper. Move the paper back and forth until the circle of 
light projected upon it is smallest and brightest. The distance 
of the paper from the center of the lens is its focal length. 

Exp. 191. In a darkened room hold a candle-flame a few feet 
in front of the convex lens, and behind the lens a screen of 
white paper or white cotton cloth. Move the screen back and 
forth until a distinct inverted image of the flame is formed on 
the screen. Measure the respective distances of the image and 
object from the lens ; also obtain as nearly as possible corre- 
sponding dimensions of the image and object, and verify the 
following formula : — 

q_a 

I~cf 
in which represents a given dimension of the object, and I a 
corresponding dimension of the image, and d and d' the respec- 
tive distances of the object and image from the lens. 

Exp. 192. Place a convex lens facing a window, and at a 
considerable distance from it. Upon a screen behind the lens 
project a distinct image of the window-frame. Observe that the 
distance of the image from the lens is greater than its focal 
length. Carry the lens nearer to the window ; the distance 
of the image from the lens increases. Verify the following 
formula : — 

i+i-l 

o % f 



56 LABORATORY EXERCISES. 

in which o and i represent the respective distances of the object 
and image from the lens, and / its focal distance. 

Exp. 193. Hold a convex lens at a considerable distance from 
the window, as before ; ascertain the distance of the lens from 
the window and from the image on the screen, and calculate its 
focal length from the last formula given. 

Exp. 194. Look with one eye through a bi-convex lens at a 
piece of engineer's paper (divided into small squares) placed at 
its focus ; with the other eye look at a piece of the same paper 
placed at a distance of ten inches, and determine how many 
squares seen with the naked eye are contained in one seen 
through the lens, and thus judge approximately of the magni- 
fying power of the lens. 

Exp. 195. Repeat the last experiment, using, in place of the 
lens, a visiting-card pierced by a pin-hole. 

THE RAINBOW. 

Exp. 196. Fill a glass bulb about 1^ in. in diameter (those 
furnished for air-thermometers answer the purpose) with a fil- 
tered solution of common salt in water. Cover the aperture of 
the porte-lumiere with a black cardboard, so as to completely 
exclude the light from a darkened room. Cut a hole in the 
center of the cardboard of the same diameter as the bulb, and 
allow a circular beam of light to pass through it and also 
through a hole of about 4 in. diameter in the center of a 
white cardboard about 2 ft. square, and strike the bulb placed 
at a distance of about 2 ft. in front of the white cardboard. A 
miniature rainbow will be reflected back from the bulb upon the 
screen around its aperture. Any spot on the screen where red, 
for instance, appears, means that an eye situated at that point 
would see red in the glass bulb. Every other color, unless 
the eye was moved, would require another bulb in the proper 
relative position. 



57 



AFTER-IMAGES. 



Exp. 197. Admit light into a darkened room by means of a 
porte-lumiere through a slit about l""" wide, and project a bright 
spectrum upon a screen. Let the spectators look steadily at the 
spectrum for five or ten minutes ; then suddenly cut off the light, 
and. at the same instant, turn on a gas light, and a reversed 
spectrum will be seen on the screen, i.e., the complementary 
of the former colors will be seen as an after-image. 

Exp. 198. Prepare a circular disk of tin 10 cm in diameter; 
punch holes about 4 mm in diameter equal distances (say from 3 
to 4 cm apart) from one another in a circle within about 5 mm of 
the edge of the disk. Cut a hole in the center of the disk about 
7 mm diameter, and place it on the spindle of the rotating appa- 
ratus. Place the disk in the path of a beam of light introduced 
into a dark room by a porte-lumiere, and beyond the disk a lens, 
and focus the holes on a screen. Rotate slowly : a flickering 
light will be produced on the screen. Rotate rapidly, and the 
light will appear steady, due to persistence of vision. 

EFFECT OF CONTRAST. 

Exp. 199. On a sheet of white paper place an opaque ball, 
e.g., a base-ball. Darken a room, and admit a small quantity 
of indirect sunlight through a space in one window about I 11 ™ 
wide. Place the paper and ball so that a shadow of the ball 
will be cast upon the paper. On the same side of the ball that 
the shadow is place a kerosene flame, at a short distance, so 
as to cast a shadow on the opposite side. So regulate the posi- 
tion of the paper and ball that the two shadows will have about 
the same depth. The shadow cast by sunlight will be yellow ; 
that cast by the flame, blue. Explain. 

Exp. 200. Obtain a strip of cardboard about 40 cm x 6 cm , and 
a pan of vermilion water-color pigment. With a camel's-hair 
brush, by repeated washes, paint a portion of the strip at one end 
about 6 cra wide very deep, so that it will appear quite dark. Then 



58 LABORATORY EXERCISES. 

gradually grade the depth of the color up from this end toward 
the other end, leaving a portion about 6 cm wide at one end un- 
colored. Let the grading be so neatly and carefully done that the 
eye will not detect the lines of separation of the different grades. 
This may be effected partly by varying the number of washes, 
and partly by thinning the washes with water. The effect is 
pleasing to the eye, and the phenomenon is instructive, as the 
effect of different depths of the same color are plainly depicted. 

Now with the same pigment paint a piece of plain white paper 
of uniform depth throughout, and about the same as the inter- 
mediate depth on the cardboard. Cut out of this paper circles 
about 15 mm in diameter, and paste them about 5 cm apart, cen- 
trally and lengthwise across the cardboard. Although all the 
circles have the same depth of color, they will appear, as the 
effect of contrast, to be of very different depths. To make 
the deception apparent, it is only necessary to take another 
strip of cardboard (or paper) of the same size as the first, cut 
holes about 13 mm in diameter, to correspond with the circles, and 
lay it over the first cardboard so as to conceal all but the circles, 
when the latter will all appear to be of uniform depth. 

Exp. 201. Get papers of as great variety of colors as possible, 
and cut out of each squares of 6 cm edge. Also cut circles of 
l cm diameter out of the same colors, two of a kind. Place two 
circles of the same color upon squares of different colors, and it 
will be difficult to persuade yourself that the circles have the 
same color until you remove the circles from the colored squares 
and place them side by side. By many experiments verify the 
following 

Rule: 

If we surround one color with another color, the former icill 
apparently be changed, as if some of the complementary of the 
latter had been mixed with it. Or, ichen any color of the chro- 
matic circle (Fig. 282, Physics) is brought into competition or 
contrast with any other color, the former is driven farther from 
the latter in the circle. 



LIGHT. 59 

In case two colors are brought into juxtaposition and there 
is not great inequality in their areas, the two colors mutually 
drive each other apart. This may be shown by placing the 
squares in juxtaposition. Also slip one square behind the 
other so as nearly to conceal one by the other. In all these 
experiments it is best to stand at some distance from the objects 
under examination. 

Exp. 202. Introduce into the porte-lumiere in place of a slide 
a green glass, having any Resign cut out of opaque paper 
pasted on it, and a black design on a green ground will appear 
on the screen ; but on bringing another light into the room or 
turning up the gas, the black design will at once appear as a 
brilliant pink. A. glass of any other color may be used, and a 
design of its complementary color will appear. 

Exp. 203. Paint figures on a white cardboard with chrome 
yellow. Illuminate the card in a darkened room with a salted 
Bunsen flame. The figures nearly disappear. Explain. 

Exp. 204. Pour a little blue coloring solution into a glass 
beaker or large test-tube, and place behind it a black cloth ; the 
larger portion of color disappears. A white cloth or white paper 
brings out the color more strongly. Explain. 

Exp. 205. Lay a piece of gold leaf on a piece of glass, and 
look through it at the sun. Explain the change in the color of 
the gold leaf. 

POLARISCOPE. 

Exp. 206. Remove the analyzer A, Fig. 293, Physics, from 
the polarizer, and examine light reflected from the top of a var- 
nished table. Rotate the analyzer, and see whether the light 
appears equally bright in all positions. Observe whether the 
color of the wood and its grain is seen better in some positions 
than in others. 

Exp. 207. Place a coin under several plates of glass, and allow 
a strong light to fall upon it. Examine the reflected light with 
an analyzer, and see whether in those positions in which the 
light is polarized the coin is visible. 



60 LABORATORY EXERCISES. 

COLOR-BLINDNESS. 

Exp. 208. Fill one of the tanks accompanying the porte- 
lumiere with a solution of sulphate of copper, and look through 
it at various colored objects, and you will get an approximate 
idea of the appearance of things to a red-blind person. Or, 

Exp. 209. Darken a room, and with a porte-lumiere introduce 
a beam of light, causing it to pass through the tank, and after- 
wards through the convex lens. Let the light fall upon colored 
objects. Colored glasses may be used. When a yellow glass 
is used, the condition of the spectators is analogous to that of 
violet-blind persons, or of those who examine colors by gas- 
light, which is deficient in violet. 

THE TELESCOPE. 

By reference to Figs. 269 and 295 of the Physics, it will be 
seen that the distance of the image (ab) formed by a convex 
lens (e.g., the object-glass) from the lens is greater for near ob- 
jects than for objects farther off . Hence the eye-glass must be 
slightly farther from the object-glass in viewing near objects 
than in viewing objects farther away. 

It will be seen that the distance of the image from the object- 
glass is somewhat greater than its focal length. On the other 
hand, the eye-glass must be brought somewhat nearer the image 
than its focal length. Hence the distance between the two 
lenses is nearly the sum of the focal lengths of the lenses. 

Exp. 210. Place the lens accompanying the porte-lumiere (or 
a lens having a focal length of 8 to 12 inches) from 4 ,n to 8 m 
from a window. Mount a white cardboard, and project upon it a 
distinct image of the window-frame. On the side of the card- 
board opposite the image write a word with pen and ink. Take 
a convex lens of two inches to four inches focal length, and 
locate it so that the writing on the card can be seen through it 
distinctly and much magnified. Now withdraw the card, and the 
image of the window-frame will be seen much magnified. 



LIGHT. 61 

In a darkened room a candle (or better, a gas or kerosene) 
flame may be substituted for the window-frame. 

Use eye-glasses of different powers. 

Prof. Crawford suggests the use of the projecting lens of a 
lantern for the object-glass, and for the eye-glass a simple hand 
magnifying glass, and adds, that, "he finds that as a rule stu- 
dents have most remarkably vague notions as to the functions 
of a telescope, and such an experiment," as indicated above, 
" helps to clear up their ideas." 

Exp. 211. Provide a white card 10 cm long and 6 cm wide, rule 
it with ink widthwise with heavy lines l cm apart, number the hues 
from bottom upward with plain figures, and place it in a vertical 
position on a black background with lines horizontal; At a 
suitable distance, say from three to eight rods, look at the card 
with the right eye through a telescope, focusing it so that the 
lines and figures can be distinctly seen. Then look at the 
card with the naked left eye, bring the image seen with the 
naked eye and the magnified image so that the adjacent vertical 
edges touch, and their lower edges coincide, measure the hight 
of the former on the latter by means of the numbered lines. 
The hight H of the larger image (H— 10) divided by the 
hight h of the smaller image will give the magnifying power 
m of the telescope ; that is 

H 

m = — . 
h 

Exp. 212. Ascertain the focal lengths F and /of the object- 
glass and eye-glass ; then 

the magnifying power m'=— . 

It will be found that m and m' are nearly equal in value. Why, 
if you use different eye-glasses (/) with the same object-glass 
(F) , do you obtain different magnifying powers ? 

Exp. 213. Allow sk}^-light to pass through the telescope and 
form a distinct image of the object-glass on a white screen held 
at a suitable distance from the eye-glass. The diameter of the 



62 LABORATORY EXERCISES. 

object-glass, divided by that of the image, is the magnifying 
power of the microscope. 

Exp. 214. Determine by the first method described above the 
magnifying power of opera-glasses, looking at a card with one 
eye through one of the cones and at the same card with the other 
eve naked. 



EXPERIMENTS WITH THE PORTE-LUMIERE. 

The following experiments are intended to be supplementary 
to those contained in Dolbear's invaluable work on the "Art of 
Projection." 

Exp. 215. Introduce a horizontal beam of light into the dark- 
ened room. Place a table so that its top will be from five to 
ten inches below the beam, and on the table in the path of the 
beam place the lens, and on the opposite wall or screen will be 
projected a large circular field of light. Place an ordinary ster- 
eopticon slide in the slide-holding disk. Move the lens back 
and forth until a distinctly-defined image is formed upon the 
screen. See Fig. 268, Physics. 

Exp. 216. Introduce the disk with ^ inch aperture. On the 
screen will appear several overlapping circles of light, illus- 
trating in an interesting manner multiple reflection (Physics,, 
p. 343). 

Exp. 217. Cut small holes of triangular, square, and other 
shapes in pieces of cardboard, and cover the aperture of the 
porte-lumiere. The image of the sun projected upon a distant 
screen in every case is round, i.e., it is independent of the Bhape 
of the hole. (See Deschanel's " Natural Philosophy," f 683.) 
Now interpose the lens. 

Exp. 218. Place the disk a (Fig. 13) witli adjustable slit in 
position, and in the path of the beam and at a distance from 
the slit about equal to its focal length, the lens /, and at a dis- 
tance of 2 m to 4 In from the lens, a screen s about l m square. 
Focus the slit upon the screen, and then interpose the bisulphide 



63 




of carbon prism p about 8 cm in front of the lens, and move the 
screen so as to receive the spectrum, as at s', preserving the 
same distance from the prism. Rotate the prism slowly on its 
axis until at least 15 to 20 dark lines are 
seen in the spectrum. More lines may be 
obtained bj* using more prisms, so as to 
increase the dispersion. 

Caution. — The cement which holds the 
glass sides on the metal frame of these 
prisms is insoluble in bisulphide of carbon, 
but is soluble in water : hence the latter 
liquid should never be used in these prisms. 

Exp. 219. Cover the disk of 1 inch aper- 
ture with coarse lace or punctured card 
paper, such as is used for book-marks, and 
project interference phenomena. 

Exp. 220. Project the phenomenon illus- 
trated in Fig. 257, Physics. 

Exp. 221. Project the piece of glass used in the last experi- 
ment, and rotate it on its longest axis. In certain positions the 
light will be intercepted by total reflection, and a deep shadow 
cast upon the screen. 

Exp. 222. Fill a small test-tube, about G™* 1 in diameter, with 
water, and project it upon the screen. Only a very narrow line 
of light will succeed in passing through the tube. Fill one of 
the tanks which accompany the porte-lumiere with water, and 
thrust the tube into the tank. The water in the tank will cor- 
rect the refraction, except that produced by the glass tube. 

Exp. 223. Thrust an empty tube into the water-tank. 

Exp. 224. Thrust a tube filled with bisulphide of carbon into 
the water-tank. 

Exp. 225. Make a saturated solution of sulphate of zinc, and 
half fill a tank with the solution. With a pipette carefully fill 
the tank with pure water. Let drop one or two drops of bisul- 
phide of carbon into the liquid. The last liquid should float 



Fig. 13. 



64 LABORATORY EXERCISES. 

midwa}', partially immersed in the two liquids. Account for 
the man}' interesting phenomena. 

Exp. 226. Repeat the last experiment, using bisulphide of 
carbon colored with iodine. 

Exp. 227. Pass a beam of light through the lens ; strike to- 
gether two blackboard brushes, just above the light, after it 
emerges from the lens, so as to render visible the cone of rays 
with its base on the lens, also the cone of rays whose vertex is 
at the focus and base on the screen. The smoke from touch- 
paper (see foot-note, p. 278, Physics) may be used to advantage. 

Exp. 228. Repeat the last experiment, intercepting a portion 
of the light by using the circular 1 inch aperture. 

Exp. 229. Introduce in front of this, or a smaller aperture, a 
concave lens, and show the divergence caused b}' the lens. 

Exp. 230. Using a small aperture, introduce a small beam of 
light, and reflect it, by means of a mirror, into different parts 
of the room. Explain why the angle which the reflected beam 
makes with incident beam is double that which the incident 
beam makes with the mirror. Explain why the spot of light 
thrown upon the walls is usually much elongated in one direc- 
tion. 

Exp. 231. Move a white screen slowly back and forth through 
the foci of the different colors produced by the lens, and observe 
the effects of chromatic aberration. Observe that the red rays 
have their focus furthest from the lens. 

Exp. 232. Mix lamp-black with French polish, and thin it 
with spirits of turpentine. With a camel's-hair brash apply a 
coating to a slip of glass. Apply other coatings as fast as it 
becomes dry until the glasfc becomes quite opaque. With the 
point of a penknife blade draw on the glass any designs or dia- 
grams which 3'ou may desire to project. 

Exp. 233. Remove the mirror of the porte-lumiere, and intro- 
duce the disk with half inch aperture. Inverted images of the 
landscape toward which the window is directed will be formed 
on the walls of the room. Persons passing by the window, at 



LIGHT. 65 

suitable distance from it, will appear walking feet upward. Let 
persons from a distance run toward the window, and observe 
the effect on the size of the images. 

Exp. 234. Cover the orifice of the porte-lumiere (its mirror 
being removed) with the lens. Behind the lens move a piece of 
white cardboard slowly from the lens until a distinct inverted 
image of a distant house, tree, etc., is projected upon the card. 
These experiments should be performed at such time of the day 
as the surfaces turned toward the lens are illuminated by the 
direct rays of the sun. 

Exp. 235. Pour dilute sulphuric acid into the "miniature 
battery," and project upon the screen. A volley of hydrogen 
bubbles will appear to fall (because the batteiy will necessarily 
appear inverted on the screen) from the zinc rod. Connect the 
electrodes, and instantly the larger portion of the bubbles will 
escape from the copper rod, a few still escaping from the zinc. 
None are seen to pass through the liquid from zinc to copper. 

Exp. 236. Place in position the "movable slide" to show 
wave-motion ; draw the slip with wavy edge past the disk with 
parallel slits, and long lines of light will play up and down on 
the screen,. while the wave will traverse the screen horizontally. 

Exp. 237. Take the two slips with wavy edges furnished with 
the above apparatus ; place them so that crest will touch crest, 
and project upon the screen. A series of elliptical areas of light 
extend horizontally across the screen ; the vertical diameters 
represent the intensity of the resultant of two similar sets of 
waves when the} T so interfere that the same phases of both coin- 
cide. Gradually move the upper slip horizontally, letting it rest 
all the time on the upper edge of the lower slip until the crest 
of the upper fills the trough of the lower slip, when all the light 
will be cut off from the screen. The gradual narrowing of the 
luminous areas, until the light is entirely extinguished, will show 
all the results of partial neutralization by interference until the 
opposite phases exactly concide, when there is a complete de- 
struction. 



(36 LABORATORY EXERCISES. 

Exp. 238. To show the toughness of liquid films by projec- 
tion, arrange the porte-lumiere for vertical projection, as de- 
scribed, pp. 40-44, Dolbear's " Art of Projection." Over the 
condenser rest a shallow transparent vessel containing a solu- 
tion of saponin e, one part of the powder to sixty of water. 
The depth of the liquid may be 4 or 5 min or less. 

Construct a wooden frame-work like Fig. 14 ; the length ah 
being such that it ma}- enclose the glass vessel. Through 
the uprights bd and ac are holes e and /, through which a piece 
of glass tubing gli is passed, fitting so as to rotate with slight 
friction. The exterior diameter of 
this may be 4 or 5 mm . Around the 
middle m is wrapped a fibre of unspun 
silk, whose lower end is fastened to 
the middle of a piece of steel needle 
about 2 cm or 3 cm long. Magnetize 
this, and let it hanff horizontallv oyer 

Fig. 14. . to J 

the middle of the saponine solution 
close to the surface. Focus so that a distinct image of the 
needle is formed on the screen. Bring a bar-magnet near, from 
side to side, and observe that the needle oscillates freely in 
obedience to it. Now turn the glass gh till the needle rests on 
the surface of the liquid without being immersed. Repeat the 
use of the bar-magnet. The needle fails to oscillate, or oscil- 
lates but slightly, being held tightly by the surface film. Again 
depress the needle until wholly immersed. Repeat the use of 
the bar-magnet. The needle oscillates freely, though slightly 
impeded by the viscosity of the liquid. Only at the surface is 
it held approximately immovable. 

Note. The author is prepared to furnish all appurtenances necessary 
for vertical projection, including a tank with a plate-glass bottom. 




LAWS RELATING TO ELECTRIC CURRENTS. 67 



LAWS RELATING TO ELECTRIC CURRENTS. 

I. To produce a current there must be two points at different 
jwtentials separated from each other by a resisting medium. 

To produce a continuous current these points must be main- 
tained at different potentials. A current is an apparent trans- 
ference of electricity from one point to another to produce 
equalization of potential. 

II. The difference in potentials between different points of a 
circuit varies as the resistances betiveen the respective pjoints. 

It is found experimentally, by measuring with a delicate 
electrometer, that, between an}- two cross-sections A and B of 
a homogeneous wire, in which a uniform current is kept flowing, 
there exists a difference of potentials, and that if the wire be of 
uniform section throughout, the difference of potentials is in 
direct proportion to the length of the wire between the cross- 
sections. 

Suppose that A, B, and C represent consecutive points in a 
circuit, d the difference in potentials between A and B, and r 
the resistance between the same points, d' the difference in 
potentials between points B and (7, and r' an unknown resist- 
ance between the same points, then 

d : cV : : r : r', whence r' = 

d 
' ' The potential of a point " is the difference between the 
potential of that point and that of the earth reckoned as a zero. 

III. Ohm's Law. The strength of a current is directly pro- 
portioned to E.3LF., and inversely proportional to resistance. 
This relation is generally expressed 

(7 = |,or(7=- E 



' B* r + R* 

when it is important to separate the internal from the external 
resistance. It will be seen that the foregoing law is only an 



68 LABORATORY EXERCISES. 

application to a specific case of a universal fact ; viz., that an 
effect is directly proportional to that which tends to produce it, 
and inversely proportional to that which tends to oppose it. 

IV. Resistance varies (1) as the length of conductors ; it varies 
(2) inversely as the areas of cross sections of conductors or the 
squares of diameters of cylindrical conductors ; it varies (3) 
inversely as the specific resistances of the substances used for con- 
ductors. ■ 

V. E.M.F. depends upon the nature and condition of the sub- 
stances which compose the battery, and is independent of the size 
of the plates and their distances apart. 

VI. Where there are no leakages the strength of a current is 
equal at all points in a circuit. 

GROUPING OF CELLS. 

VII. For a given battery of cells the most effective way of 
grouping them, when they are required to work through a given 
external resistance, is that in which the difference between the 
external and internal resistance is least. Hence a given battery 
works to the greatest advantage ichen the external and interned 
resistances are equal. 

It must not be inferred from the latter statement that when 
the internal resistance is less than the external resistance, that 
we should increase the internal resistance for the sake of the 
resistance. The internal resistance of itself is a positive dis- 
advantage. The necessity for so great an internal resistance 
results only from the fact that it is an unavoidable accompani- 
ment of high E.M.F. when that is obtained by connecting in 
series the requisite number of cells. 

To get the maximum current through a high resistance it is 
clear that we will gain more by the increase of E.M.F. resulting 
from connecting cells in series, than we will lose by the accom- 
panying increase of resistance, since the latter is but a small 



LAWS RELATING TO ELECTRIC CURRENTS. 69 

part of the whole. If the external resistance is very small so 
that the resistance of the cells is a large part of the whole, we 
shall gain very little by arranging them in series, since we 
should increase the total resistance of the circuit in nearly the 
same ratio as we should the E.M.F. If we arrange them 
abreast, we shall neither gain nor lose in E.M.F. , but we shall 
reduce rapidly the total resistance of the circuit. 

VIII. The current will be a maximum when the number of 
cells connected abreast is numerically equal to Vnr-r-R. 

Thus, suppose the number of cells is 40, and r = 3 ohms, and 
E = 8 ohms, then V40 x 3 -=- 8 = 3 +', nearly 4 ; hence, four 
cells should be connected abreast, and the ten groups connected 
tandem. 

The following law as given by Gordon will be found very 
convenient. 

To obtain a maxim ion current, the ratio of the number of cells 
in series to the number of cells connected abreast should equal the 
ratio of the external resistance to the resistance of a single cell. 

V R Y 

That is, C is a maximum when — = — , or when M = —r. in 

n r n 

which N is the number of cells in series and n the number con- 
nected abreast. 

LAW OF DIVIDED CIRCUITS. 

LX. Wlien a circuit is so constituted that at some point there 
are two paths open to the circuit, the current will divide between 
these two paths in the inverse ratio of their resistances, and the 
joint resistance of the two paths will be neither the sum nor the 
difference of their respective resistances, but ivill be expressed by 
the product of these resistances divided by their sum. 

ESTIMATING WORK DONE BY A CURRENT. 

X. TJie mechanical work of a current may be calculated from 
the following formula, in ichich C is the current strength, and V 



70 LABORATORY EXEKCISES. 

is the difference in potential of the terminals of the instrument 

in which the work is done : — 

C amperes x V volts . ~ 7 . 7-7 

i = rate of doing ivork in horse-powers. 

745 

For example, to find the rate at which actual work is con- 
sumed in an electric lamp, measure the whole current in am- 
peres ; measure the difference of potential (with an electrometer) 
between the terminals of the lamp in volts ; multiply them to- 
gether, and divide by 745 ; the result will be the number of horse- 
powers used up in the lamp. That is, a current of C amperes 
falling V volts will perform, in passing through the instrument, 

. t. .1 fi f C amperes x V volts\ , 
work at the rate of x[ = — — = ~ rz ] horse-powers. 



745 J 

XI. The amount oj chemical decomposition produced by a cur- 
rent in a given time varies as the strength of the current. On this 
principle is constructed the voltameter, which measures the 
strength of the current by the amount of chemical action it effects. 

XII. The weight in grams of an element deposited by elec- 
trolysis is found by multiplying its electro-chemical equivalent (i.e., 
the atomic weight divided by the valency) by the strength of the 
current in amp&res, and this product by the time in seconds dur- 
ing which the current electrolyzes. 

A current of one ampere strength will decompose about three 
grams of water per hour. A current of n amperes will decom- 
pose 0.0000973 n grams of water per second. 

XIII. The number of units of heat developed in a conductor 
is proportional, (1) to its resistance; (2) to the square of the 
strength of the current; and (3) to the time the current acts. 

A current of one ampere flowing through a resistance of one 
ohm, develops therein 0.00024 calorie of heat per second. Hence, 

H (calories) = C 2 (amperes) x R (ohms) x t (seconds) x 0.00024. 

That portion of a current not expended in external work is 
; ' frittered down into heat," either in the battery or in some 
part of the circuit, or in both. 



LAWS RELATING TO ELECTRIC CURRENTS. 71 

The absolute amount of heat generated by the oxidation of a 
given quantity of zinc is perfectly constant ; but it may be dis- 
tributed in various proportions between the battery and the 
external circuit. 

Whenever the current heats a wire, produces decomposition, 
or performs work of any kind, each of these acts is accomplished 
at the expense of the heat in the batter}*. If the current turn an 
electro-magnetic engine which pumps water, or lifts a hammer, 
the battery loses heat by these mechanical acts. The precise 
amount of heat thus lost is restored by the falling of the 
water, or of the hammer. So also when water is decomposed 
in a voltameter, the battery loses an amount of heat equal to 
that which would be produced by the recombination of the 
separated oxygen and hydrogen. 

Faure corroborates these results in a series of careful exper- 
iments, in which he interposed increased resistance by increas- 
ing the length of the wire connecting the two ends of the battery. 
His main results are given in the following table : — 

Length of Internal Heat Heat outside Total. 

Wire. of Battery. the Battery. 

(Units.) (Units.) (Units.) 

25 ....... . 13127 4965 18092 

50 11690 ....... 6557 18247 

100 10439 ....... 7746 .... ... 18185 

200 8992 9030 18022 



72 LABORATORY EXERCISES. 



LAWS OP THE ELECTRO-MAGNET. 

I. In order to produce the most advantageous effect, the resist- 
ance of the helix of an electro-magnet should be equal to that of 
the portion of the circuit not included in the helix, i.e., to the 
rest of the circuit. When several electro -mag nets are used in 
the same circuit, the sum of the resistances of all the helices should 
be equal to the resistance of the rest of the circuit. 

Caution. Having decided correctl}* the resistance an electro- 
magnet should have, it is possible to make a serious error in 
selecting wire either too fine or too coarse. For example, sup- 
pose that an electro-magnet of 4 ohms resistance is to be made. 
If No. 20 wire is used, it will require about 170 yds. ; but if No. 
32 wire is used, it will require only 16 yds. The latter would 
give only a few convolutions, which might not produce the 
maximum magnetic effect ; while the former might be so coarse 
that the outside layers would have little effect on the core ; 
hence the maximum effect in neither case would be obtained. 
In electro-magnets of ordinary size the best distance for the 
outside layer from the core is between three-eighths and half an 
inch. Or, in general, 

II. The thickness of the helix should be equal to the diameter 
of the core. 

It is apparent from the above laws that we would choose for 
a short circuit, or for a circuit where there is little other resist- 
ance, an electro-magnet of small resistance, i.e., one made of 
coarse wire. On the other hand, for a long circuit, or a circuit 
of high resistance, an electro-magnet of high resistance, i.e., 
one made of fine wire, should be ordered. Not because high 
resistance of itself is advantageous (it is a positive disadvan- 
tage) , but in order to make the most of the existing current, 
weakened by the other high resistance in the circuit, we require 



LAWS OF THE ELECTROMAGNET. 73 

many turns of wire, the resistance which it brings with it being 
a* necessary but an unwelcome adjunct. "The condensed reason 
whv we use fine wire — and a great deal of it — for circuits of 
high resistance, is that the high resistance of the circuit greatly 
enfeebles the current, and we must use fine wire to make the 
best of the remaining strength of the current by a greatly - 
increased number of convolutions." 

Why is not a magnet containing many convolutions of fine 
wire — in other words, a high resistance magnet — as efficacious 
ia producing magnetic effects when in a circuit of low resistance 
as when in a circuit of high resistance ? It would be equally 
efficacious if, by introducing the high resistance magnet, the 
whole resistance of the circuit in both cases were proportion- 
ately increased, and thereby the current proportionately de- 
creased. Take an example. Suppose the whole resistance of 
a circuit including that of the battery is 20 ohms. Then a 
magnet suited to this circuit should have a resistance of 20 
ohms. The introduction of this magnet will double the resist- 
ance of the circuit and reduce the current strength one-half. 
But if the entire resistance of the circuit is 1 ohm, and a magnet 
of 20 ohms' resistance is introduced, the entire resistance of the 
circuit will become 21 ohms, and the current is reduced to one 
twenty -first its former strength. In the latter case, the advan- 
tage derived from the great number of convolutions of wire in 
the magnet cannot compensate for the great reduction of 
current. 

Roughly, " the total length of the core, including both arms 
and the back armature or connecting yoke, should be about 
eleven times the diameter. When, however, the circuit is long 
and the electric source is of feeble energy, the magnet should be 
long and of small diameter. When, on the contrary, the circuit 
is short and the current strong, the core should be of large 
diameter." 

" The attraction of magnets for prismatic armatures at a 
distance is greatest when they are presented flatwise : but 



74 LABORATORY EXERCISES. 

when in contact, the attractive force is greatest when they are 
presented edgewise." — Fiske. 

Electro-magnets with short cores charge and discharge more 
rapidly than those with long ones. Advantage is taken of this 
in constructing telegraph sounders and relays. 

III. The attractive force exerted by an electro -mag net is pro- 
portioned to the diameter of the core and to the square root of 
its length. 

IV. The attractive force of electro-magnets is proportional to 
the square of the strength of current for a like number of convolu- 
tions, and to the square of the number of convolutions for like 
strength of current. 

If the strength of the current (acting on the electro-magnet) 
and the number of convolutions in the helix vary at the same 
time, which is nearlv always the case, since by increasing the 
number of convolutions without changing the battery we in- 
crease the resistance of the circuit, and thereby weaken the 
current, 

V. The attractive force of the electro-magnet is proportional 
to the square of the strength of the current multiplied by the square 
of the number of convolutions. 

VI. The maximum of saturation depends solely upon the mass 
of iron contained in the electro -mag net irrespective of its form. 

VII. The maximum degree of magnetization, of which a mass 
of soft iron is susceptible under the influence of the electric cur- 
rent, is more than five times as great as that which a correspond- 
ing mass of hardened steel is capable of retaining. 



ELECTEICAE EXITS ADOPTED IX PRACTICE. 75 



ELECTRICAL UNITS ADOPTED IN 
PRACTICE. 

Resistance. The resistance offered by 37.8 m (41.3 yds.) of 
pure copper wire, size No. 20 (New British standard TVire 
Gauge), diameter, 0.914 ram (0.36 in.), temperature 15° C, is 1 
ohm. The megohm is 1,000,000 ohms. 

Tlie legal ohm is the resistance of a column of mercury 106 cm 
long and l qmm in section at 0° E. 

Potential, electro -motive force. The difference of potential 
between the plates of a Daniell or Gravity cell, or the electro- 
motive force of one of these cells, is about 1 volt. The volt is 
the difference of potential or electro-motive force necessary to 
sustain a current of 1 ampere strength against a resistance of 
1 ohm. 

Current strength. A current flowing in a wire of resistance 
1 ohm, between the two ends of which a difference of potential 
of 1 volt is maintained, is the unit of current, or current 
strength, and is called an ampere. It is a current of 1 coulomb 
per second. 

Quantity of electricity. The amount of electricity conveyed 
in 1 second by a current of 1 ampere is called 1 coulomb. It is 
the quantity that would charge a condenser of 1 farad capacity, 
under an E.M.F. of 1 volt. 

Electrostatic capacity. The capacity of a condenser, which 
would contain 1 coulomb of electricity when charged by an 
electro-motive force of 1 volt, is 1 farad. A microfarad is a 
millionth part of a farad. 



76 LABORATORY EXERCISES. 

Rate of doing work. The rate at which work must be done 
in maintaining a given current against a given resistance is 
measured by EC, in which E is reckoned in volts and C in am- 
peres. Hence, rate of doing work, the power of a current, or 
the rate at which work is given out in a circuit, is measured by 
its EC. The unit employed is called a watt, or volt-ampere, and 
is the power developed by 1 ampere falling 1 volt. 

Work done. The work done in 1 second, when the rate of 
working is 1 watt, or the work obtained by letting down 1 
coulomb of electricity through a difference of potentials of 1 
volt is 1 joule, or volt-coulomb. 



EQUIVALENTS OF WORK. 



0.101937^. 
0.737324 ft. lb. 



r 0.10 

1 joule or volt-coulomb = i .00024067 caTorie of heat. 

I 10 7 ergs. • 
lkgm _. 9. 81 j 0U ie S . i f t . lb. = 1.35626 joules. 

EQUIVALENTS OF POWER. 

r 0.00134059 horse-power. 
0.101937 k s m per second. 
1 watt or volt-ampere = { 6.11622**° per minute . 

0.000240670 calorie per second. 
0.144402 calorie per minute. 
k 10 7 ergs per second. 
1 horse-power = 745.941 watts; 1 ft. lb. per second = 1.35626 watts ; 
lkgm p er se cond = 9.81 watts. 

SOURCES OF ENERGY. 
Primary Sources. 

1. Primordial energy of chemical affinity. 

2. Solar radiation. 

3. Energy of the earth's rotation. 

4. Internal heat of the earth. 



ELECTRICAL UNITS ADOPTED IN PRACTICE. 



A. Potential. 



B. Kinetic. 



Secondary Sources. 

r 1. Combustibles. 

2. Food of animals. 

3. Ordinary water-power. 

4. Tidal water-power. 

5. Winds and ocean currents. 

6. Hot springs and volcanoes. 



CLASSIFICATION OF VARIOUS FORMS OF ENERGY. 



I. Mechanical Energy, 



II. Molecular Energy 



A. Visible kinetic energy ; i.e., energy of a body 
in visible motion. 

B. Potential energy of visible arrangement ; e.g., 
a stone elevated above the earth. 

C. Kinetic energy of electricity in motion. 

D. Kinetic energy of radiant heat and light. 

E. Kinetic energy of absorbed heat. 

F. Molecular potential energy; e.g., the so- 
called " latent heat." 

G. Potential energy of electrical separation. 
H. Potential energy of chemical separation ; e.g., 

oxygen and hydrogen. 
A + B + C + D, etc., = a constant quantity. 



78 



LABORATORY EXERCISES. 



TABLES OF REFERENCE. 



SPECIFIC GRAVITY OF VABIOUS SUBSTANCES. 



Brick, common . 



Acetic acid 1.060 

Alcohol 0.792 

Aluminium, sheet .... 2,670 

Antimony, cast 6.720 

Ash, dry 0.690 

Ash, green 0.760 

Asphalt 2.500 

Basalt 2.950 

Beech, dry 0.690 

Bell-metal 8.050 

Birch 0.690 

Bismuth, cast 9.822 

Bisulphide of carbon . . . 1.293 

Boxwood 1.280 

Brass, cast 8.400 

Brass, sheet ...... 8.440 

j from 1.600 

[ to 2.000 

Carbon, gas 1.760 

Carbonic acid, liquid . . . 0.830 

Cedar, American .... 0.554 

Cedar, Lebanon 0.486 

Cedar, West Indian . . . 0.748 

Cedar, Indian 1.315 

Chalk 2.330 

Chestnut 0.606 

Clay 1.900 

Coal, anthracite 1.600 

Coal, bituminous .... 1.270 

Cobalt 8.800 

Concrete, ordinary .... 1.900 

Concrete, in cement . . . 2.200 

Cork 0.240 

Copper, cast 8.607 

Copper, sheet 8.780 



Earth 



Deal, Norway 0.689 

Diamond 3.530 

( from 1.520 

( to 2.000 

Ebony 1.187 

Elm 0.579 

Elm, Canadian 0.725 

Ether 0.716 

Eeldspar 2.600 

Fir, spruce 0.512 

Firestone 1.800 

Glass, flint 3.000 

Glass, crown ...... 2.520 

Glass, common green . . . 2.520 

Glass, plate 2.760 

Gold 19.360 

Gold, 20 carats 15.700 

Granite 2.650 

Gun-metal (10 cop., 1 tin) . 8.561 

Gutta-percha 0.966 

Gypsum 2.286 

Human body 0.890 

Hydrochloric acid .... 1.200 

Ice at 32° 0.930 

Iron, cast, average .... 7.230 

Iron, wrought, average . . 7.780 

India-rubber 0.930 

Iodine 4.950 

Ivory 1.820 

Lead, cast 11.360 

Lead, sheet 11.400 

Lignum vitae 1.333 

Lime, quick 0.843 

Limestone 3.180 

Logwood .,,..,, 0.913 



TABLES OF REFERENCE. 



79 



SPECIFIC GRAVITY OF VARIOUS SUBSTANCES. - Continued. 



Specific 
gravity. 

Magnesium 1.750 

Mahogany, Honduras . . . 0.560 

Mahogany, Nassau .... 0.668 

Mahogany, Spanish . . . 0.852 

Maple 0.675 

Marble 2.720 

Mercury 13.596 

Milk 1.032 

Mortar, average 1.700 

Muriatic acid 1.200 

Naphtha 8.470 

Nitric acid 1.217 

Oak, American, red . . . 0.850 

Oak, American, white, dry . 0.779 

Oak, live, seasoned .... 1.068 

Oak, live, green 1.260 

Oil, linseed 0.940 

Oil, olive 0.915 

Oil, turpentine 0.870 

Oil, whale 0.923 

Phosphorus 1.830 

Pine, red, dry 0.590 

Pine, white, dry 0.554 

Pine, yellow, dry .... 0.461 

Pine, pitch 0.660 

Pitch 1.150 



Platinum, average .... 21.531 

Plumbago 2.267 

Quartz 2.650 

Rock salt 2.257 

Saltpeter 1.900 

Sand, quartz 2.750 

Sand, river 1.880 

Sand, fine 1.520 

Sand, coarse 1.610 

Silver 10.474 

Slate 2.880 

Sulphur, natural 2.033 

Sulphuric acid 1.840 

Tallow 0.940 

Tar 1-010 

Teakwood 0.806 

Tile, average 1.830 

Tin, cast 7.290 

Water, 32° 0.999 

Water, 212° 0.958 

Water, distilled, 39° . . . 1.000 

Water, sea 1.627 

White metal (Babbitt) . . 7.310 

Willow 0.400 . 

Zinc, cast 7.000 



Air, 32° 1.0000 

Ammonia 0.5367 

Carbonic acid 1.5290 

Carbonic oxide 0.9670 

( from 0.3400 
I to 0.6500 

Chlorine 2.4600 

Hydrochloric acid .... 1.2540 



Coal gas 



gravity. 

Hydrogen 0.6930 

Marsh gas 0.5596 

Nitrogen 0.9714 

Oxygen 1.1057 

Sulphuretted hydrogen . . 1.1912 

Sulphurous acid 2.2474 

Vapor of water 0.6235 



80 



LABOKATORY EXEKCLSE*. 
AVERAGE COMPOSITION OF ALLOYS. 





-u~ - 

7/2 ;",. 




pq 


5! 


I 

c 
- 



If 


Is 

II 


= 

n 


if 

"of 


! 
s 


5 

i 
5 




1 
7 


Gold . . 
Silver . / 
Copper ) 
Tin . . 
Zinc . . 
Lead . . 
Antimony- 
Arsenic . 
Bismuth . 
Nickel 


92 

8 


93 

7 


69 
31 


75 
3 

19 
3 


85 
15 


60 
30 

10 


93 

7 


55 
27 

18 


50 
50 


80 
20 


3 

80 
17 


25 
25 

50 


9s 
2 



FRIGORIFIC inXTURES FOR THE ARTIFICIAL PRODUCTION OF COLD. 

Note. To obtain the following results, the temperature of the ingre- 
dients must be reduced previously to the given temperature by some of 
the other mixtures. The last four mixtures yield the temperatures given 
whatever may be the previous temperature of the ingredients. The tem- 
peratures given are of the Fahrenheit scale. 





Proportional parts, by weight, in the mixture. 


Water 

Sal ammoniac . . 

Nitre 

Common salt . . . 
Nitrate of ammonia 
Sulphate of soda . . 
Carbonate of soda . 
Phosphate of soda . 

Potash 

Muriate of lime . . 
Snow or pounded ice 
Diluted nitric acid . 

" sulphuric acid 
Temp, of ingredients 

" of the mixture 
Cold produced . . 


16 
5 
5 

8 

+50 

+4 

46° 


1 

1 
1 

+50 

-7 
57 


5 
6 

4 

+50 
-14 
64 


6 

9 

4 

+50 
-21 
71 


1 

1 

+32 



32 


s 

10 

-<;s 
-91 
23 


5 
4 

-40 
72 


4 
3 

+32 
-51 
83 


1 

2 

-5 


1 

2 

5 
-12 


5 

5 

10 

24 

-18 


5 
5 

12 
-25 



TABLES OF REFERENCE. 



81 



TABLE OF LINEAR DILATATIONS OF SOLIDS. - Stewart. 



Name of Substance. 


Length at 100° C. of 
a rod whose length 
at 0°C. = 1.000000. 


Name of Observer. 


Glass (tube) .... 




1.000776 


Rov and Ramsden. 


Copper 




1.001722 


Lavoisier and Laplace. 


Brass 




1.001867 


" " 


Iron, soft (forged) . . 




1.001220 


a 


Steel (untempered) . . 




1.001079 


<c it 


" (tempered yellow) 




1.001240 


« 


Cast iron 




1.001072 


Daniell. 


Lead 




1.002848 


Lavoisier and Laplace. 


Tin 




1.001767 
1.001951 




Silver 






Gold (standard of Paris, 
annealed) .... 


not I 


1.001552 


Lavoisier and Laplace. 


Platinum 




1.000884 


Dulong and Petit. 


Zinc 




1.002976 


Daniell. 



RELATIVE THERMAL CONDUCTIVITY OF METALS. 



Silver 100.0 

Copper 73.6 

Gold 53.2 

Brass 23.6 

Tin 14.5 



Iron 



11.9 



Steel 11.6 

Lead 8.5 

Platinum 8.4 

Palladium 6.3 

Bismuth . . . . . . . 1.8 



SPECIFIC HEATS. 



Lead 0.0314 

Iron 0.1140 

Glass 0.1900 

Gold 0.0324 

Copper 0.0951 

Brass 0.0940 

Platinum 0.0324 

Silver - . 0.0570 

Zinc 0.0955 



Tin 0.0562 

Ether at 17° 0.5160 

Alcohol at 17° 0.6150 

Quicksilver 0.0333 

Oil of turpentine at 17° . . 0.4260 

Water at 0° 1.0000 

"Water mean between 0° and \ 
100°. ..... 



> 1.0050 



82 



LABORATORY EXERCISES. 



CROSS -SECTION OF ROUND WIRES, WITH RESISTANCE AND WEIGHT OF 
PURE COPPER WIRES, ACCORDING TO THE BIRMINGHAM WIRE GAUGHX 
— Grat. 

Temperature 15° C. 



B.AV.G. 


Diameter. 


Area of Cross-section. 


Resistance. 


Weight 
(density = 8.95). 


No. 


Ins. 


Cms. 


Sq. Ins. 


Sq. Cms. 


Ohms per 
Yard. 


Ohms per 
Meter. 


Lbs. per 
Yard. 


Grms. per 
Meter. 


0000 


.454 


1.1530 


.1620000 


1.0444000 


.000152 


.000167 


1.884000 


934.7000 


000 


.425 


1.0790 


.1420000 


.9150000 


.000174 


.000190 


1.651000 


819.1000 


00 


.380 


.9650 


.1130000 


.7320000 


.000217 


.000238 


1.320000 


654.8000 





.340 


.8640 


.0908000 


.5860000 


.000272 


.000297 


1.056000 


524.2000 


1 


.300 


.7620 


.0707000 


.4560000 


.000349 


.000382 


.822000 


408.1000 


2 


.284 


.7210 


.0633000 


.4090000 


.000389 


.000425 


.737000 


365.8000 


3 


.259 


.6580 


.0527000 


.3400000 


.000468 


.000512 


.613000 


304.2000 


4 


.238 


.6050 


.0445000 


.2870000 


.000554 


.000606 


.518000 


256.9000 


5 


.220 


.5590 


.0380000 


.2450000 


.000649 


.000709 


.442000 


219.5000 


6 


.203 


.5160 


.0324000 


.2090000 


.000762 


.000833 


.377000 


186.9000 


7 


.180 


.4570 


.0254000 


.1640000 


.000969 


.001060 


.296000 


146.9000 


8 


.165 


.4190 


.0214000 


.1380000 


.001150 


.001260 


.249000 


123.5000 


9 


.148 


.3760 


.0172000 


.1110000 


.001430 


.001570 


.200000 


99.3000 


10 


.134 


.3400 


.0141000 


.0910000 


.001750 


.001910 


.164000 


81.4000 


11 


.120 


.3050 


.0113000 


.0730000 


.002180 


.002380 


.132000 


65.5000 


12 


.109 


.2770 


.0093300 


.0602000 


.002640 


.002890 


.109000 


53.9000 


13 


.095 


.2410 


.0070900 


.0457000 


.003480 


.003800 


.082500 


40.9000 


14 


.083 


.2110 


.0054100 


.0349000 


.004560 


.004980 


.063000 


31.2000 


15 


.072 


.1830 


.0040700 


.0263000 


.006060 


.006620 


.047400 


23.5000 


16 


.065 


.1650 


.0033100 


.0214000 


.007430 


.008130 


.038600 


19.2000 


17 


.058 


.1470 


.0026400 


.0170000 


.009330 


.010200 


.030700 


15.3000 


18 


.049 


.1240 


.0018900 


.0122000 


.013100 


.014300 


.022000 


10.9000 


19 


.042 


.1070 


.0013900 


.0089400 


.017800 


.019600 


.016100 


8.0000 


20 


.035 


.0889 


.0009620 


.0062100 


.025600 


.028000 


.011200 


5.5600 


21 


.032 


.0813 


.0008040 


.0051900 


.030700 


.033500 


.009360 


4.6400 


22 


.028 


.0711 


.0006160 


.0039700 


.040000 


.043800 


.007160 


3.5500 


23 


.025 


.0635 


.0004910 


.0031700 


.050200 


.054900 


.005710 


2.8300 


24 


.022 


.0559 


.0003800 


.0024500 


.064900 


.070900 


.004420 


2.1900 


25 


.020 


.0508 


.0003140 


.0020300 


.078600 


.085800 


.003670 


1.8200 


26 


.018 


.0457 


.0002540 


.0016400 


.096900 


.106000 


.002960 


1.4700 


27 


.016 


.0406 


.0002010 


.0013000 


.123000 


.134000 


.002340 


1.1600 


28 


.014 


.0356 


.0001540 


.0009930 


.160000 


.175000 


.001790 


.8890 


29 


.013 


.0330 


.0001330 


.0008560 


.186000 


.203000 


.001540 


.7660 


30 


.012 


.0305 


.0001130 


.0007320 


.218000 


.238000 


.001320 


,6630 


31 


.010 


.0254 


.0000785 


.0005070 


.314000 


.343000 


.000915 


.4540 


32 


.009 


.0229 


.0000636 


.0004100 


.388000 


.424000 


.000746 


.3670 


33 


.008 


.0203 


.0000503 


.0003240 


.491000 


.536000 


.000585 


.2900 


34 


.007 


.0178 


.0000385 


.0002480 


.641000 


.701000 


.000442 


.2200 


35 


.005 


.0127 


.0000196 


.0001270 


1.260000 


1.370000 


.000229 


.1130 


36 


.004 


.0102 


.0000126 


.0000811 


1.960000 


2.150000 


.000146 


.0728 



TABLES OF REFERENCE. 



83 



THE FOLLOWING TABLE EXHIBITS THE DECLINATION OF THE NEEDLE 
IN DEGREES FOR A SERIES OF DECADES AT DIFFERENT POINTS ON 
THE NORTH AMERICAN CONTINENT. THE PLUS (+) SIGN PRE- 
FIXED TO A NUMBER INDICATES A WESTERN DECLINATION, AND 
THE MINUS (— ) SIGN AN EASTERN DECLINATION. 



Year 

(Jan. 1) 


Halifax, 

N.S. 


Cam- 
bridge, 
Mass. 


New 
York 
City. 


Wash- 
ington, 
D.C. 


Erie, 
Pa. 


New 

Orleans, 

La. 


San 

Francisco, 

Cal. 


Sitka, 
Alaska. 


1700 




+ 9.80 


8.50 












1710 




9.20 


8.00 












1720 




8.70 


7.60 






-3.40 






1730 




8.30 


7.20 






-3.70 






1740 




7.90 


6.60 






-4.10 






1750 


+ 12.5 


7.50 


5.90 






-4.70 






1760 


13.0 


7.20 


5.20 






-5.30 






1770 


13.7 


7.00 


4.60 






-5.90 






1780 


14.4 


6.90 


4.40 






-6.50 






1790 


15.1 


6.90 


4.29 


-0.10 


-0.70 


-7.00 


- 12.80 




1800 


15.9 


7.10 


4.28 


0.00 


-0.70 


— 7.50 


- 13.40 


-26.12 


1810 


16.7 


7.50 


4.30 


+ 0.30 


-0.60 


-7.90 


- 13.90 


-27.11 


1820 


17.4 


8.00 


4.47 


0.60 


-0.30 


-8.10 


- 14.42 


-27.89 


1830 


18.1 


8.64 


4.91 


1.00 


+ 0.03 


-8.20 


- 14.92 


-28.48 


1840 


18.7 


9.33 


5.59 


1.49 


0.44 


-8.14 


-15.38 


-28.88 


1850 


19.3 


10.03 


6.34 


1.99 


0.91 


-7.94 


-15.78 


-29.08 


1860 


19.8 


10.67 


6.96 


2.47 


1.39 


-7.61 


-16.11 


-29.08 


1870 


20.1 


11.51 


7.43 


2.90 


1.87 


-7.15 


-16.36 


-28.88 


1880 


+ 20.3 


+11.63 


+ 7.84 


+ 3.26 


+ 2.31 


-6.62 


-16.52 


-28.50 



Note. "The west declination of the magnetic needle at Cambridge, 
Mass., for the beginning of 1884 is + 11.766 3 , and increased annually, at 
the epoch 1880, 0.0347°. The minimum west declination at this place 
occurred about the epoch 1782, 0.7°. The magnetic dip at this place is 
74°." — E. C. Pickering, Harvard College Observatory. 



MEAN INDICES OF REFRACTION AND DISPERSIONS OF SEVERAL 
SUBSTANCES. 





Index of 
Refraction. 


Dispersion. 


Crown glass (mean) 

Flint glass (mean) 


1.530000 
1.600000 
1.336000 
1.372000 
1.680000 
1.540000 
1.000294 


0.0220 
0.0420 
0.0132 


Alcohol 

Carbon disulphide 

Canada balsam 

Air 


0.0133 
0.0837 



003 651 462 




$MM 



